So to take this back to the actual topic of the thread, sexual themes in RP, I admit to not really understanding how and why some things are allowed, some things are banned, some things are stigmatized more than others, etc. Just to get it out of the way first, so that I can go into other points and hopefully people won't focus on this one specific aspect -- re: rape RP? Eh. I can take it or leave it. It's not personally my bag, but I can see ways in which it can be used to further someone's storyline in a way that they find enjoyable, especially in regards to being pressed into a situation worthy of 'adapt or suffer'. It could make an excellent Awakening to Pandaemonium, if that's what the player wanted, or a fantastic trauma for the First Change to occur. If this is what you wanna get out of RP, then this is what you wanna get out of RP. If you have someone willing to help you with that story, more power to you. But it should probably be at least moderately consent-based if it's just a random physical act with no character development options agreed upon by the character. And that is the extent of my opinion on that particular line of thinking. Ergo, moving on to the things I find more interesting!
NOW, for the more interesting stuff, which I will likely break up into a couple of different posts for reasons of length:
There are plenty of other things that sexual content can be used for in RP, and not all of them are just writing porn on screen. Now, before we go further, let's assume the standard MU line that:
All players agree that they are eighteen years of age or older and understand that Mature themes and content are going to follow from this game:
With that out of the way, let's talk about some of the ways that sexuality could be used in a game.
As a character development tool - Believe it or not, this can be done. This can be done to fantastic effect. I've taught entire arcane philosophies (the philosophy of the Adamantine Arrow lends itself particularly well to this) through the use of sex, tension, and the exchanges of energy, with or without the accompanying magic and drugs. It's completely do-able. Hell, it's more enjoyable, more personal, and more spiritual in that sort of setting, IMO, because it stresses the fact that there are facets to the occult that stray outside of the readily obvious, connections that can be made between Thing A and Thing B in ways that can inspire both pleasure and pain, passion and fear. So, to respond to @Thenomain's thing earlier, you absolutely can do this and still be mindful of the Abyss. You can even use it as a way to teach about the Abyss. And you can do it in ways that don't even remotely border on the Abyss. It all depends on what you want to inspire.
As a means to an end - Sex can be used, in story, in a wide variety of settings. Perhaps your character is not above using sex to blackmail another person with the videotape that you made of it. Perhaps your character likes you pull the honey trap maneuvers and use it to leverage your own goals into place. Perhaps your character really needs some sort of escape, or you have some sort of hedonistic vice that lets you benefit from this while exploring your character's psyche.
Whatever the case, you can surely benefit from this type of RP, and given how geared the WoD has been toward sexual themes, even strange and deviant sexual themes like the girl that basically dry humps her brother while feeding off of him (true story. Vampire 2.0, I'm looking at you), so incorporating any of these into your own RP should not be stigmatized. It's just as valid a part of the game as any other, and seems to even be actively encouraged because of the mood that they can evoke. At the end of the day we're all adults. We are at least old enough to be able to find Pornhub. The fact that someone gets a blowjob on screen, or poses a supernaturally sensual encounter, is not beyond the realm of reason.