I really, really don't care.
I really, really think you do.
From the way you've described it, your colleague has not been talking about his sex life. I mean, yeah, I agree with the general principle that colleagues should not be talking about whoever they shagged the other night and what positions they tried, but this isn't what has happened.
If a colleague feels the need to make a big deal out of coming out, as opposed to casually slipping into conversation a non-sexual anecdote about a current/former partner who is not of the opposite sex, then I'd suggest they previously didn't feel comfortable bringing their whole self to work, and it's great that they now do. Maybe it was all in his head, and he was projecting his own insecurities, or maybe he's picked up on the attitudes of people like you who claim not to care, but actually, care quite a lot.
I'd support it, apart from anything else, there could be newer/more junior colleagues absolutely bricking it about how unsupportive they perceive their workplace to be. If someone else 'goes first', it can take a lot of the anxiety away, and again, I'm all for making people not feel on edge all the time.
If people bring their whole self to work, they're more likely to perform at their best, and commercially, isn't that what every business wants?
Think back to what the colleague has actually said - from your posts, it doesn't sound like he's said much more than 'I like men and women' - and why you're so hacked off. What's the real reason your back is up? You don't need to post there, but I'd encourage you to challenge your own bias. Sometimes we don't realise what kind of crap we're holding onto until we're put in a situation that forces us to confront our own beliefs. If you reflect on it, you can let go of the crap.