I am a member of Quora, but I don't think it is. However, as long as the question asker is writing in a proper manner, I believe it is acceptable to ask sexually oriented questions and for people to answer accordingly. There may be some questions/answers that do indeed relate to adult content. I think it's okay since there are occasions when asking online is the only way a person can learn the nuanced answer. I respond to a lot of other inquiries there, the majority of which are not of an adult kind.