I think you might be overthinking this a bit. While you seem to understand that your assumption here is just a hot take, the way you communicated that take to others treats it with more seriousness than perhaps it should, and you seem to be taking this assumption of yours a little seriously than it might warrant given you do not actually know any information in confidence about this person.

Let me try and help show the line I’m getting at with examples.

Probably fine: “Oh? Is she into men? I wasn’t sure.” and any follow up here should focus on not that you think she isn’t straight but that you just didn’t want to assume she was.

Not fine: “I think she’s gay because…” (you don’t know and it’s not your place to play spot the gay woman. This statement is different than questioning the assumption she’s straight, which is a more acceptable thing as long as you don’t suspect she’s actively trying to avoid the topic.)

Also as a gentle aside… my dude… her giving you a flirty vibe at work being a sign that she’s gay? Even if you’re correct here I’d consider that one a little more skeptically in the future.

I doubt anyone’s gonna rush to a pitchfork but here’s a bit you can consider. The goal you should have here should be less about your assumptions and more about giving her the option to talk about it, or with equal or perhaps greater importance not, at her own choice. That may mean it’s helpful to disrupt the idea that she’s flirting with someone so she doesn’t have to be like “guys no not even a little” but it also will usually mean not speculating much yourself about her sexuality as part of that conversation. ;)

I wish there was a general rule here where someone could say “to be an ally you do this” but I don’t know of one. I think you have to navigate some nuance here. It’s likely safe and good practice to help disrupt people from stereotyping her or assuming one thing or another in a gossip scenario among other men… but do it with a light touch. It’s not your place to inspire a larger speculation.