@nervyuyt - because the person that asked stated they were a man, and no women had commented regarding that issue? I'd prefer that wasn't made something it isn't, but apologies if the wording bothered you.
Quite obviously anyone can ask, but why would you, in the context of a stranger asking whether a skirt is ok, which is what has happened here; policing and intruding the OP's personal life, and whether she is taken because her ring might mark her to be - is a bit weird in this context, as well as (to me) outdated thought. Specifically as its the only real point they made, having not looked very much at her outfit at all, since they called it a dress and the title even says 'skirt'.
If you were actually dating this person in real life and the outcome affected you, then by all means you might ask if they were married or engaged if a piece of jewellery made you suspect they were ( though as i said, these days i'm not sure it quite works that way for a lot of people) rings go on fingers, not 'ring fingers' -people wear them as such for a variety of reasons that are not matrimony or engagement.
Either way that is not what's happened here, the person that commented isn't dating the OP are they, they just want to ask what she is doing out and about dating with a ring on that 'ring finger' - for some reason
which i think is a bit off. You are of course, welcome to think that it isn't.
I hardly think OP is on here having extra marital dates and has just been sprung by eagle eyed commenters. Though again if she were, since she asked for outfit advice only , I'm not sure what it would have to do with a total stranger that they'd point it out.
I'd have found it just as odd if the comment was ' nice 'dress' OP but mainly i'm concerned about who's bag that is, are you dating but really that is your boyfriends bag there hmm' - obviously i'm being trite but its a bit insidiously problematic when men police or involve themselves in things like this unsolicited, obviously just saying the outfit was nice and leaving it there would have sufficed. It would also be strange if a woman said it, though none here have done so, so i have not called any out.