I think this is common for both genders to think, its a way to convince yourself to not talk to someone when really you have no idea if they are taken and you are just afraid of putting yourself out there. I am guilty of this! I will also tell myself he is probably gay.lol
When I first meet a guy, if they're at all attractive, I assume he's taken. If I get to know him and he's flirty, I would change my initial assumptions. I guess it's just something I do. Maybe it's to keep from getting hurt, I dunno.
If a guy never sees you on weekends and texts or calls either really late at night or in the day or takes days to reply and is generally "hard work" I assume he's got someone on the go and communicating around avoiding her! Happened to me twice and can spot this pattern now thankfully!
I had a cashier tell me she loved my wedding ring (it wasn't a wedding band, was on the wrong hand and wrong finger, too)
I think everyone else would believe I'm single; I keep that info private; though on occasion I've gotten clients who have invited my wife and I out to parties.
The kids I teach are blown away when I say I'm not married and have no kids. They always ask; I'm assuming that it's because they want to figure out how old I am as opposed to thinking I'm SO AWESOME that I HAVE to be married.
They assume a guy is taken since they don't have the time to see if he really is since time is short, and they'll just pick up on signals.