my girlfriend wanted a [threesome] with another guy, but i didn't want to do it, because i was scared me and the other guy would be bumping plums
when two men are having sex with a woman at the same time (one vaginally and the other anally), and their nut sacks hit one another
"my girlfriend wanted a [threesome] with another guy, but i didn't want to do it, because i was scared me and the other guy would be bumping plums" What is the definition of "bumping plums"?