In the Monster Girl Encyclopedia world, monster girls are made as well as born. Monsterization is the umbrella term for a whole host of different processes by which men, women, corpses, plants, animals, elements, gods and various inanimate objects are all transformed into cum-guzzling nymphomaniacs.
A whole section of Fallen Maidens is devoted to case studies of different monsterization classes; see there for in-depth case studies on yuri, Demon Realm mana corruption, roper/slime-carrier/matango parasitism, dark slime melting, necromancy, the hand of god, snake pits, cursed items, alpification, and not eating properly.
The generic case
Most people who get monsterized don't want to be monsterized, because the vast majority of people who undergo the process are female Order humans, and the Order teaches that anything that happens in relation to monsters is bad  apart from zealous religious persecution. There are exceptions, of course; sorceresses may pursue witchdom or lichdom  in their quest for magical power, and there are some heretical sects which hold monsterization in a positive light . Still, in the vast majority of cases, monsterization is a process violently resisted by those subjected to it. On becoming a monster, the fear and loathing a woman felt towards her own transformation does not necessarily melt away immediately. Elves and valkyries are notoriously conflicted about acting on their new irresistible impulses, and in Baphomet Complex, Lilia Flimlight's misgivings persist for hours even after she sleeps . But regardless of whether they're happy about it or not, their newly ravishing body is gripped by a throbbing, needy lust for male company. "Women who’ve only just become monsters still have their human values, but with the monstrous body that’s hot for men, and the intense feeling of hunger, the human mind can’t hold out. Driven by desire, they end up attacking human men ". It's at this point that those who had someone they truly loved as a human make a beeline towards their "husband"; for those who didn't, the first man they spot is likely to be attacked for their first semen meal.
The physical changes of rockin' tits, flawless sensitive skin, and more importantly, thirty foot tails or an ass the size of a wagon, therefore do not prevent the immediate use of those attributes for sexy times. It is not clear to how many of the monsterization methods or monster girl species this applies, but the succubus family at least tends to have their transformations go via a two-step process. First, the spirit energy is replaced with demonic energy, producing a partially-transformed lesser succubus. These creatures have protective fur over parts of their bodies, as lesser succubi have a monster's sensitivity but haven't yet acquired the monster's tolerance to supercharged pleasure; being groped too vigorously during semi-transformed sex is a sure-fire way to make them ahegao themselves unconscious . Lesser succubi must then rape their way to enough spirit energy to complete the transformation, and become a fully-fledged succubus.
The metaphysical aspect
To the casual bystander, monsterization usually involves a woman becoming substantially more attractive and then raping that bystander. But from a magical standpoint, all the significant stuff happens invisibly. The fundamental difference between being a monster and not being a monster is that monsters have demonic energy inside them  and non-monsters don't. If a female is exposed to enough demonic energy, it snuffs out her ability to produce spirit energy; and the point at which this ability is destroyed is the point at which there is no chance of recovery. With only demonic energy left inside her, and no way to replenish her mana autonomously, milking men's semen for spirit energy is from then on her #1 source of magical sustenance. The reason that men can't be transformed into monster girls by this very same process is that men's autonomous spirit energy generation is much faster, and gets faster still in the presence of demonic energy, so it can't be snuffed out except in very peculiar circumstances. Or by death, after which point demonic energy could do unspeakable things to your bones .
Former humans who are transformed by sex tend to form a special bond of respect with the monster that transformed them. Both Lilia Flimlight and Wilmarina Noscrim come to view their lilim rapist as a second mother , and go on to work for them as subordinates. Those who have been transformed also tend to be more keen to transform others, as they know first-hand the thrill of monsters' sexual "liberation".
The first and largest instance of "transformation into a monster girl" was the one that enraged the Chief God and produced the Monster Girl Encyclopedia world we know today. It was the great magical work that turned the skull-crushing, blood-drinking monsters of the former Demon Lord into the pelvis-groping, semen-drinking monster girls of the current Demon Lord.
So this wasn't so much a "monsterization" as a "succubusization", from one kind of monster to a different, hybrid kind. Just as humans who are transformed into monsters rapidly begin to prefer their new forms for all the mind-shaking pleasure it brings, the monsters transformed into monster girls all seem to like it this way too (though the ones in Zipangu don't like it quite enough to contribute to the Demon Army ). Exactly what happened to the formerly male members of the monster species is not known .
Early in the current Demon Lord's reign, the depraved dark elves appealed to the succubi for super-hedonistic bodies too, and that race quickly became entirely monster. The dwarves went the same way, albeit somewhat slower and with less intentional planning. The Fairy Kingdom races are on the path to monsterization after the Sabbath decided to aggressively proselytize amongst them . In the current era, transformation into a monster girl is mostly accomplished by the various one-on-one methods mentioned previously: getting yuri raped by a succubus, having a parasite egg stuffed up your barren vagina, being mauled by a werewolf, etc. Although it isn't stated outright, it doesn't seem like any non-monsters could be converted into monsters before the current era; otherwise, the dark elves (and dwarves) could have asked the succubi to turn their women into sexy sluts in any of the previous ages.
- Early settings info, The Order: For that reason, they are fiercely hostile towards the monsters which drown in pleasure and corrupt humans. They claim that “The monsters are evil.” and in many of the churches, they hide the truth about the monsters and are spreading the lie, “Monsters kill and eat people,” among the humans.
- Witch profile: Originally they were human women but they sought “eternal youth” and “extreme power.”
- Fallen Maidens, CASE STUDY Ritual: Along with their god echidna, they hold monsters in an entirely positive light. They desire the power to bind what they want to themselves the way that monsters, especially lamia family monsters can. Human girls brought up that way since early childhood strongly desire to become monsters themselves and acquire a beautiful, alluring snake body.
- Baphomet Complex, Section 2: "My words gave birth to terror and despair. If anyone found out that I'd been turned into a monster, people would certainly come to suppress me at once. Of course, I wouldn't be able to stay in this town anymore. If that happened, I probably wouldn't be able to be with big brother anymore, either."
- Lesser Succubus profile
- Fallen Maidens, Monsterization Physical Changes
- Fallen Maidens, Monsterization Physical Changes: "If one has sex with that body, the amount of pleasure received is many times greater than the pleasure that can be obtained by having sex with a human body. In the case of human women, the pleasure is so powerful that their mind cannot process it and they'd pass out"
- Usually this means having just demonic energy inside them, but there are marginal cases. Monster elementals have elemental magic too; incubi have spirit and demonic energy; and very small quantities of demonic energy can be tolerated by females without snowballing into monsterization.
- Skeleton profile: Zombies and other undead type monsters can fundamentally never rise from a man’s corpse; however, in the skeleton’s case, magical energy doesn’t dwell within the body, the body itself is made up of magical energy. They are an exception, and men’s bones can become monsters too.
- Baphomet Complex, Section 4: "At the sight of the person who, in a certain sense, could be called my benefactor—no, my mother—I raised myself on the bed and smiled sweetly."
- MGE-1, The Utopia the Demon Lord Dreams Of
- MGE-1, The Beautiful Yokai: Although they don't serve the demon lord directly, it is thanks to the demon lord that they've been changed into their present forms, enabling them to live even more closely with humans. Since the monsters of Zipangu are extremely happy about this, none of them have bad opinions of the demon lord.
- And the fate of the monster males is a question of some pathos amongst English fans. Did the males get magically converted to females? And if so, did your baphomet waifu used to be a hulking, muscular devil-goat behemoth with a demon ram's cock the size of a python? Nobody knows!!?1!
- MGE-1, What Comes Next for the Fairy Kingdom: Lately, they've been proactively having a lot of cultural exchange with the organization sabbath (p.184) that baphomet (p.180) leads. The fairies are being indoctrinated in various methods of using their small, childish bodies for the sake of further enjoyment with human men, and getting to feel even better.