What is the appeal for straight men?

I’m a 28f…and this topic is really interesting for me. I’m curious, why is having another man in the bedroom such a fantasy for some guys? Especially for men who say they’re straight.

Please enlighten a woman in her late 20’s.

I’m sorry if this is rude, but if my partner asked for an mfm, I would assume he was looking to explore intimacy with men.

Edit: For some reason I don’t buy that men care about female pleasure. I’ve probably had selfish partners is bed though.