Felching is the act of sucking semen out of an anus or vagina. It can be accompanied by “snowballing”, where the semen is shared between people orally. Felching can be done by both straight and gay partners. Felching is also relatively common in the gay male barebacking community. A recent study found roughly one in sex men who bareback also do felching.
There is relatively little information about felching in the academic literature. The greatest risk with felching or snowballing is the potential to spread STDs, including HIV and hepatitis. These diseases are spread most often from the inserting partner to the receptive partner. There is also a risk of spreading intestinal parasites, if the receptive partner has one. For that reason, it’s considered a “high risk sexual behavior”.
There is no way to use barriers to reduce risk with felching. The best way to protect oneself is to get all potential sexual partners STD tested before felching. Keep in mind that HIV can take up to three months to show up on an HIV test, so you may wish to wait until a test at three months is clean.