Very nicely put Quath.
My first and foremost feeling on sexuality is that it should do no harm. Of course, "harm" can sometimes be in the eye of the beholder. For example, I am a proponent of legalizing prostitution, as I believe a woman (or man) should have the legal right to make money by selling of themselves and, with legal regulations, there would be less disease and violence attributed to it. However, the idea of married or coupled people going to prostitutes without the knowledge of their partners is morally offensive to me. But then one could make the case these cheats would do it anyway. So it becomes a round and round issue.
I have a gut reaction to the idea of relatives sharing a sexual partner. But that is my reaction, and has no bearing on the actual incidence. It's just that I have three siblings and there is an "ewww" factor I cannot help. My sister has dated one of my exes, years later, and that didn't bother me as much. But at the same time? Ewww.
Anything done to children, animals, or done without the full knowledge and consent of all involved and causing harm (including your comment about STDs) I find vile and would happily be at the head of that witch burning committee.