Well, if religion has confused you, then I have created a sufficiently plausible religion.
Oh, but this question is not about religion. And it is much more complicated than it seems. It is about the difference between the modern perception of adultery and rape and what was characteristic of ancient societies.
Today, adultery is almost universally considered a misdemeanor, and rape is a crime. In ancient societies, this was not like that. Or not quite like that.
In ancient societies, adultery (especially by women) was a crime. Rape, on the other hand, could be considered a misdemeanor (in the case of unmarried women or slaves belonging to others). The severity of the punishment also varied depending on the victim's and the offender's social statuses. In such societies, there was no police force, and it was unwise to seduce or rape a woman from a wealthy or simply large family. This could provoke a blood feud.
As for why female infidelity is a big deal, but male infidelity isn't. Or, to be more precise, not always. It's a question of inheritance. And the lack of DNA tests. The husband's family (the inheritance line) had to be sure that his wife's children were his and part of their kin. However, this could also be a problem for a man if he seduced the wife of someone powerful enough. It could even lead to execution. Whether it was by consent or not was a matter of little importance. And it depended more on the desired interpretation for the accuser.
And that's just the tip of the iceberg. If anything, Selene is a young, unmarried woman without the protection of her family. Although in one of Origins, she has a family she can rely on, but they are far away. So, not involving other guys in cheating and not sleeping with everyone is a matter of morality, but it is dictated more by society's expectations than by religion. There is the goddess of sex and fertility in the local pantheon. So the problem is not sex itself.
Finally, how Selene feels when she does things that aren't exactly moral is up to you, the players. She can be a good girl. Or not. And feel no remorse whatsoever. Or do.
Your choice.