Porn games usually don't feature multiplayer because jerking off, especially to incest porn, is something people prefer to do in private, when nobody is watching. While there are some couples who watch porn movies together, there are likely considerably less couples who play porn games together, because playing a game and having sex at the same time is something only the great Cthulhu would be capable of.
When it comes to online games, where you can find people for e-RP however, you can do that in:
World of Warcraft: Shitty graphics but a large community and some people who like e-RP/cybersex of either gender.
Aion: You can have lingerielike armor and characters that fill every porn star with envy. I don't know if the American community has some people who are into e-RP
Black Desert Online: Pretty much the same as for Aion, only with even better graphics.
GTA Online: Strippers, night clubs and even a surprisingly large roleplaying community. Maybe you will find people who like e-RP here as well, though I think women are a minority here.
In any case, when it comes to cybersex and e-RP: Do it only, if you are 100% sure, that the other person is an adult. Also be aware that much like casual sex, emotions can get quite real with all the possible consequences of real sex except for one. There is more to e-RP than just glorified phone sex.