I’ve seen people parrot this around so much, but I have never been presented with a source or any evidence to corroborate it. I think it’s just a myth at this point.
Some mamonos are definitely married. Signs of marriage are clearly visible in Atlach-Nacha, Mindflayer, Night Gaunt and, it seems, Wendigo too. These mamonos transform their husbands into parts of themselves. For example, in Atlach-Nacha, her spider part is actually her husband...
As Sir Tort states, yes. There are certain Mamonos whose images shown (may it be main or accompanying image) are of individuals that are married. Basically, this is KC showing us what they are likely to look like when married. It doesn't take much to tone-down their appearance or take some extra details off to know what they look like when un-married (or single).