Cambridge Dictionary defines "thwarting" as the action of a gay male putting his testicles into a another gay male's anus and deliberately attempting to catch HIV; or, when a gay male puts his entire mouth, teeth and all, into the anus, to eat out the HIV.
In the case of gay thwart sex incest, it is combined with convential homosexual fornication, between blood-related family members.
This is a fact, according to Yahoo Answers user Billie - a general practitioner of 38 years.
Normal person - You're ugly.
Gay person - FUCK YOU. Try do what you want, i will fucking gay thwart sex incest you