Other people might find it gross but it is considered hot for a lot of people, especially gay men who are practically cum addicts! This is the practice where two gay men transfer the cum in their mouth to another numerous time until one of them drinks every droplet of it.
It is done regardless of any gender or whose cum it actually is. Usually, this is done after a good blowjob where the mouth is the recipient of the semen, as the other man continuously gives the other a good head until he finally orgasms in his tongue and mouth-he does not swallow it yet, letting it simmer in his tongue and passes it to his partner, a taste of his own self that was released by the former. Sometimes nobody has to swallow it and instead they make out and the cum is shared on their mouths like melting ice cream. This is a good ending to good and kinky sex.