friend with benefits
noun [ C ] informal uk/ˌfrend wɪð ˈben.ɪ.fɪts/ us/ˌfrend wɪð ˈben.ə.fɪts/
性伴侣，炮友
a friend with whom you also have a sexual relationship
They're not a couple - I guess you'd describe them as friends with benefits. 他们不是一对儿，我猜你会以为他们是炮友。