Give me some sensible perspectives on this. I'm single. About 18 months ago a married (no DC) male colleague who I'd always been mates with started spending a lot more time socialising with me, being friendlier than he had in the past, very subtle flirting. I knew his marriage had been in trouble for a while (I'd heard him telling male friends) but, even though I fancied him, I deliberately didn't flirt back and avoided situations where anything could develop. Eventually things went back to the way they had been before and I assumed he had sorted things out with his wife.
Fast forward to this summer. New female work colleague joins office and male colleague starts behaving towards her in the same way he did to me. But she doesn't back off and instead flirts with him, invites him round her house and makes plenty of opportunity to spend time alone with him. Now he is leaving his wife for her and I'm gutted (flame me for this) that I didn't respond when I had the chance.