It's an interesting topic, and I have thought about it before.
In short, no I do not consider them immoral. Just "different".
See, Mamono simply have different morals than us, and by their own standards, they're not evil at all. They consider killing, hurting, maiming, etc. horrible things, just like us. Perhaps even more so. But they also consider disloyalty towards family something unthinkable, while we consider rape to be something unthinkable.
However, the thing is, Mamono do not rape out of malice like humans do. Mamono see rape as a mating ritual, and usually simply do not have any other choice. Like the Reptile King said, Mamono that actually do get the chance to get a man peacefully will try to adapt to that. Not to mention that to Mamono, sex is not just something they do for pleasure or reproduction, like humans. They also need it to feed. To survive. Denying them it purely out of predjeduce is outright cruel. So it is understandable that they are forced to take what they need.
Not to mention they aren't hypocrites about the rape thing. They might like raping others, but they love it even more if someone were to rape them. Again, to them it's a truly honest way of saying "I love you and want to be with you forever".
Now, another point to make is that, at their core, Mamono are hedonists, yes. But not just any kind of hedonists, rather ethical hedonists. All they truly want is love and pleasure, not just for themselves, but for everyone. And never at the cost of the love & pleasure of someone else. That is actually quite commendable. Besides, don't we all, at some deep, primal degree, want to find a soul mate to love and stay with forever? Mamono do, and are just a lot more honest about it. They just want to love and be loved, nothing else.
In a sense, all things considered, Mamono can even be argued to be morally superior to humans. I know some don't agree, even I don't fully, but from an objective point of view, they aren't that bad.
But I've droned on long enough. Aside from that, Reptile King has already said what else I could have said.