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 homosexualfornication, between blood-related family members.
This is a fact, according to Yahoo Answers user Billie - a general practitioner of 38 years.
Cindy was totally into me. I was definitely going to hit that, but Sally didn't want us to hook up, she made up all these lies about me, and gave me a genital thwart.