It's my power fantasy. I conquer women, and their families if they want me to(Brienne). I don't touch sub-par, disloyal whores(Cait). I defeat evil enemies, like my ancestors in myths. I reclaim lands, build cities, castles, and even conquer my nemesis' daughter and impregnate her. I enslave evil demons that try to do the same to me, and I pick my friends well.
Let me fuck the big wolf woman. Nobody cares if it's your OC. It's a game. The enemies can fuck my companions if I lose combat(Never happened so far), so why can't I fuck other people's wives?
Because its their precious OC and/or self insert and they are engaging in inflict type magical realm.
In inflict type magical realm the DM/GM inflicts his fetishes on the players, the players exist to be the victims of his own fetishes. the GM gets off on having his self insert do something specific to the player (often cuck them or rape them).
They literally masturbate going "real live human beings are going to play my game and my big strong self insert is going to cuck them!!!" *fap fap fap* "oh yea, fuck. my self insert is going to breed their fucking innocent little daughter and there is nothing they can do about it but cry." *fap fap fap*
If the player was able to turn things around then they wouldn't be getting their rocks off, instead they would feel like they are the one who got violated. To them it is not just "a game", it is them inflicting their fetishes on some real life victim. When you find yourself stuck in an inflict type magical realm the only winning options are not to play... or to use mods.
Incidentally the reason they raged so much about mods in their older games was exactly that. Those mods opened up their self inserts for retribution. CoC and TiTs both have their share of sacred cow SI's that were untouchable, that the "owner of the character" forbade any content that puts them in any position other than dominant / fucking over the MC. Whether that owner was a writer or a commissioner.
This has caused many player blowups over the years.