Wouldn't that make them bisexual? I mean if it's just a one-off thing then I suppose you could call that individual bi-curious but really I tend to think that once you engage in sex with someone of the same sex even once then that makes you bisexual. But I suppose that's no longer considered the case, I'm not sure.
But if you're talking about an individual who's bisexual but who no longer engages in same-sex activities because they're in a monogamous relationship with someone of the opposite sex then yes, I know someone like that very well. She's my wife. Before I met her she dated & had relationships with women as well as men but once we became involved she stopped having sex with women because we're committed to one another.
Though she still identifies as bisexual she just happens to be in a monogamous "straight" marriage with me.