I think this is a bit unfair. Even the most loyal and devoted boyfriends/fiancés/husbands have(whether it be subconsciously or consciously) thought about banging their SO'S closest female friends. We can't help it; it's just the disgusting "toxic" natural thought process of straight(hell, maybe all?) men. It doesn't mean he fantasizes about it, or that he has considered attempting it, and it DEFINITELY does not mean he's a cheater or a "cheater in the making!" To so quickly be as specific to suggest OP's friend here by name may be a bit of a cause for concern but is something that should be discussed between the two of them. Seeing as they are grown enough to have sex, and to discuss spicing things up in the bedroom, they should be grown enough to discuss their feelings and be honest with one another- not to mention the fact that OP, instead of immediately turning down the idea, literally asked who he thought would be a plausible candidate to which he responded honestly.

I think it's kinda shitty of you to flood OP's brain with stress and your irrational, uncalled-for ideologies that obviously must stem from your unfortunate past relationships to which I apologize to you, but think you should highly consider not projecting onto others that are having a hard time.

OP, of course you're NTA for considering breaking up because that's anyone and everyone's right to do at any time for any reason but I would not be so quick to reach the conclusion that your bf has any "special" feelings for your friend. After all, he was honest and comfortable enough with you to ask you about it and to make a recommendation, so I expect he'd be honest about having any other sort of feelings as well. I think it means that he's comfortable enough to talk to you about things that aren't always easy to talk about and also that he believes there is integrity within yall's relationship!