Rape RP, specifically, is an issue when it involves / spills over to people that directly did not consent to it. It is different because of the type of mental impact that dealing with this specific line of RP can do to people, and the stigma PCs who try to ignore the RP receive when they can't deal with it because if your character is such a good person, why are they not...?
Somebody who's been shot during a robbery or something can speak up on this from their perspective, but I'm going to talk about this from the perspective of someone who was raped to try and explain why having to interact with rape RP at all is a problem when my PC murdering someone or character death or murder or whatever other crimes don't bother me.
There are two types of players that play characters that are raped IC. Those who have been raped, and those that have not been raped. Those that have not been raped tend to make light of / poorly portray / say and do things that are really, really, really offensive and hurtful to some of those that fall into the latter category. Watching some idiot essentially MOCK me about one of the most traumatizing things that have ever happened in my life is not something that feels good. It's just not.
Violence is in the theme of these games. Rape (or on this vein, childhood sexual abuse), a type of violence, is not specifically called out in violence. There are many, many, many things that you can use to brutalize a character suitably and get a similar end result to whatever you want -- there is no reason to employ something that is so fucking hard for certain people to deal with. Harm to good ratio here is waaaaaaaaay in favor of harm.
If somebody had had somebody shot in front of them and it traumatized them and it was a trigger, well, violence is in the theme of the game, they know by looking at the LABEL that it is not for them. Rape? There are probably only a handful of games where rape is actively part of the theme. If it was, believe me, I won't be playing there.