If you don't put an age on your profile before you post in the MM forum, you can keep your screen name.
What you're describing is the Boyfriend Disclaimer, also known as a Proactive LJBF. The Boyfriend Disclaimer rejection occurs when a woman slips the information of her unavailability (due to the BF) covertly into casual conversation. I've covered this countless times with frustrated guys asking me why a woman would tell him she had a boyfriend in the middle of a sarge. It simply a communication methodology with the latent purpose of proactively rejecting him (where a LJBF is a reactive rejection). Men have a tendency to think women are abstract and scattered when in fact they are very calculating, it's just that covert communication is second nature for them.
The idea behind the Boyfriend Disclaimer is that you've telegraphed your interest in her overtly enough that she wants to save herself (and possibly you) the potential embarrassment and discomfort of having to needlessly go through the process of you asking her out. It's similar to single women wearing false wedding rings in order to thwart unwanted advances. As I said, it's proactive - "Hey lets save us both the trouble of you trying to hook up with me and get to the part where we agree to LJBF." This is a very common practice for exceptionally attractive women who, through frequency, more readily read the approach behaviors of guys who are attracted to her.
That said, unless there's a (real) ring on her finger, every woman is single. C&F is amazingly effective in defusing a BFD when used to draw attention to it as a tactic. Don't let a BFD thwart your sarge. Often enough it's a bluff she's not used to being called on, turn that to your advantage, but do so cleverly.