What does it mean if my boyfriend wants a threesome?

Anonymous
My boyfriend and I have been dating for about six months, and he's recently brought up the idea of a threesome. As a girl who is pretty intent on monogamy, I was a little startled - what does this mean?

Is he bored of me? Or worse, is this his way of cheating on me but with my permission?

Guys, please help me out - is a threesome something that most guys want just because they like sex? I want to know if it means anything more and if I should be concerned about my relationship with my boyfriend.
What does it mean if my boyfriend wants a threesome?
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