I'm pansexual. A whole lot of lesbians want nothing to do with me. Shitty, but it's their loss. I would disclose early on, and if it's a deal breaker, well, you probably didn't want to be with them anyway.
If as a hypothetical I was single and an AMAB man disclosed that he was bisexual, it wouldn't be an immediate "no" but it would definitely be a "proceed with extreme caution". I've been cheated on by a man who claimed he had no interest in men. A man who as is my general behavior pattern, I checked in with regularly to see if the current relationship parameters were working for him. He chose to cheat on me. He chose to not use safe sex practices. He chose to expose me to risk, because we were fluid bonded.
I tend to read threads in the other Ask a blank sections, because sometimes they're interesting. I generally don't read Ask a Bisexual Man because so many of the threads reinforce negative opinions I have about bisexual men. If you're so inclined, go take a peek. See how many are men trying to get others to give the okay/give approval for infidelity. Go see how many are men talking about cheating being okay, since it's with another man.
I know, not all men, not all bisexual men, etc. Between this site and my personal experiences, though? Not an immediate deal breaker, but I'm pretty reluctant. If the person doesn't disclose their orientation early on, it's likely to make me distrust them and then become a deal breaker because of that. I am open as heck about myself. I'm fortunate (and appreciative) of living somewhere that being LGBTQ+ isn't likely to make anyone react violently. I know it isn't the same for everyone. I'm out to my family. I respect and acknowledge that not everyone is able to be out of the closet. Having said that, it's my personal choice to not want to be involved with anyone who IS in the closet.
I'm a blunt, brutally honest person. Also, my memory can be spotty as fuck. It's so much easier and fits my code of ethics to just be truthful about shit. If someone hid their sexuality from me, as generally accepting as I am? Deal breaker. Being bisexual or pansexual, proceed with caution but not a deal breaker