Foe. And a hilariously bad liar. He was just asking if you swing because he doesn't. Uh huh. Sure he was. I imagine him scrambling and stuttering and backtracking in a frenzy and I hear the sound of people running in cartoons.
Here's what this guy is thinking: you liked him in high school, so maybe you'll still be attracted to him now, almost 20 years later. If he plays his cards right, he can get into your pants soon and liven up his dull married life--and yours--with a little sumpin'-sumpin' on the side from time to time with his new FWB. Oh, he's not looking for anything serious. He's got a wife and kids and doesn't want to upset the status quo; he just could use a little more tail in his life, preferably the kind that comes with no strings attached and not a lot of chit-chat afterward, because he really needs to get back to the office.
Since you are not looking for the same, your motives don't mesh. If you just want to be FB friends, tell him so. It might work out. But something tells me that once he realizes you aren't going to bed with him, his interest in you will go limp faster than his boner did when you told him you don't swing.
HA! I liked the sound effects. Yes, I heard those noises in my head when reading this. I think I also heard Snagglepuss saying, "Exit, stage left, right away even."
Chances are he will eventually get lucky and find someone who says yes - my husband did. Do his wife a favor and tell her what her husband is up to on Facebook. If they swing together she will already know and it won't be a big deal. If not, she deserves to know the truth about the man she is married to.
And stop flirting on Facebook. If you wouldn't say the same things in front of your husband, you are definately walking a very fine line of fidelity yourself. I am not against flirting - it is whether it is out in the open or not that is the issue.