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Sparky Well, humans are rather complex. Have you ever heard about Stockholm Syndrome? Hostages begin to to identify themself with the 'criminal'. Especially if their life is at stake, they adapt by protecting the 'criminal' as a form of self-protection.
There are many complex psychological mechanisms at work. One of them would be 'cognitive dissonance', where you mentally know that the 'criminal' is your enemy and you just help them to help yourself, but on the other side the act on physically helping them makes you 'like' them more. The more effort someone puts into something, the more he begins to appreciate it. Even if it is absolutely not logical. So time is an essential factor here.
Sects use these concepts to 'brainwash' their clients. There are some nice documentation on these subjects.
Obviously this also works when sex is involved. Probably even more easy in a society with magical superpowers. Sex wasn't always as 'holy' as it is now. At times it was restricted, at other times even very young children were mimicking adults.
This is way more complex than what I write here, but if you are interested in this stuff, just use wikipedia or read a (related) book.