I apologise if this comes out sounding mean and judgemental, because it's not supposed to be, but why does it matter to you if a FB has a wife or girlfriend? I only expect monogamy in LTRs. If you have to be monogamous with a FB, what's the point? And you said (I think, I may be misinterpreting) that you are dating other men, so why should he not date other women?
I wouldn't want an FB that is married or is in an otherwise exlcusive relationship or at least has left that impression on the other person. Maybe it's just karma, I don't want to interfer on thos relationships.
But it is also somewhat selfish as well. Here in the UK you can still name aldultering partner in divorce proceedings. I don't know the implications of that legally, but who wants that on record especially if you were unwitting partner at that.
Another reason is that I don't like being stood up and I don't like people changing plans too much. They are generally the hallmark of a married person who wants to keep it secret. A few years ago a rather fetching guy asked for my phone number. I gave it to him. He called me twice around 5:30 on weekdays, let's meet now. Umm, let's not. The one time we plan to meet, what happens, by the time I show up at the pub he tells me he can't stay long because his sister needs an emergency babysitter and she's looked everywhere for one. Well, three strikes you're out.
But reasonably speaking, even if he weren't married, why would I want to go through each day wondering if he's going to call me and ask me out. I've got lots of friends I can make plans and see instead.
Another reason, also selfish, is that given what wronged partners can get up to, once again I do not want to be a part of it.
When people are multidating, each person in that group, unwitting or otherwise, theoretically get equal weighting. If I make plans with someone on Friday, then I tell other men, Friday is not good for us to get together. If he wants to know why, well, my response would be, we need to go steady for that privilege. And I have noticed that the very guy who will ask what and with whom I am doing on Friday will most certainly dodge the question when it's put to him. Exclusivity has its reciprocal privileges.
An ideal FB arrangement would be for me no matter how often or seldom we get togther, they are firm and we have a nice time together. Since everyone is entitled to their own dealbreakers those are mine and this is why I came back to my senses and told the guy to get lost.