[Storyteller.]

I'm sorry that this coworker found out that way; even if you did want people at work to know, I imagine that this isn't the way you would have gone about it.

That said, what exactly about the situation are you finding upsetting? Do you think your coworkers may not be accepting or that you might be at risk of repercussions (such as discrimination) because of it? Or is it more that you wanted to keep this to yourself and don't really feel comfortable talking about it with people you don't know?

If you don't want it to go any further, I'd recommend talking to the coworker who saw the thread and explaining that this is something you don't want anyone else to know. If they're a decent person they'll respect that and just let it be, since it's not their secret to tell. If they haven't made a big deal about it by this point, chances are that they don't think it is a big deal. They might be so accepting that they wouldn't even twice about it because they don't assume everyone is straight, so finding out that you're gay wouldn't faze them.

I wouldn't suggest going out of your way to try to convince your coworkers that you're not gay, as that can easily backfire and make them think you are anyway. Instead, if it's something you don't want to talk about, then just... don't talk about it. You don't have to say anything about your private life that you don't want to, and it's perfectly fine to keep something like this to yourself. Not being open about being gay doesn't mean you're pretending to be straight. For example I'm not out at work, but not due to me deliberately trying to somehow pass as straight; it's just not something I've talked about, and I'm fine with people coming to their own conclusions. It doesn't bother me because I know who I am.

Ultimately it's your decision as to how you proceed. There's nothing wrong with not being "out and proud", but there might also be nothing wrong with your coworkers knowing, even if this isn't how you wanted them to find out.