I'd feel turned on by it. I've always thought that my man likes both sexes but has always hidden the fact. If he ever did I would probably pissed off until I got to taste the action for myself
He never did before he met me, but I suspected he has a bisexual curiosity, so I encouraged him to try blowing one of my FWB when we were doing a threesome together, and he did. I found it fascinating to watch, as they both gave each other a bj. David choked, and couldn't get much of it swallowed, but I gave him points for trying. He's still not ready to be butt-fucked though!
Disclaimer: I'm not in a relationship at the moment, so this is hypothetical.
I'm of the opinion that everyone has baggage. Some baggage is more important to disclose than other baggage. I don't want a detailed count of everyone my partner's been with (and I don't want to give a detailed account of my past). But some things do need to be disclosed: same sex contact that went beyond the horny curious teenage stage is something that needs to disclosed (my opinion only -- your mileage will vary).
If a guy is currently having sex with guys, I need to know that. I need to know that he's playing safe. He's going to have to play safe with me.
If a guy has had sex with guys, but isn't currently, it's less important that I know. I'd like to know, so I can say my piece about playing safe.
Is it a dealbreaker? Maybe, maybe not. If he's not going to play safe, it's a dealbreaker. Otherwise, I guess it's case-by-case.
What he did before we became a couple is no a big deal. If my husband had chose to be with a man, that was his choice then. Now... if he wants to do it now... we'd need to have a long talk.