Hypnotico saidAnyone else gets this question a lot from potential BFs? I do, and I hate it, I'm talking IRL not online.
Everyone, straight or gay, is single at some point in their lives, unless they live in cultures that practice arranged child marriages. And a single gay guy at 21 doesn't make me wonder at all, and not something I'd ever question him about.
Many guys ARE single at 21, at best doing short-term dating, learning the ropes (not necessarily THOSE ropes

), preparing themselves for the more significant LTRs to come. I find nothing out of the ordinary in this at all.
I'll tell you the ones I really do wonder about: older guys, some even closer to my age, who have never had a partner, or an LTR, some never able to keep a BF in their entire lives. And I've met these guys. Worse, I've dated a few.

And while I was never so blunt to ask them outright, I did wonder about it once I learned of it, and they usually told me anyway. Generalized answers about not being ready to settle down (with time running out on them), not finding the perfect guy yet (don't hold your breath), and being a solitary person not able to live with others (for which I felt sorry for them, almost breaking out into Streisand's "People" but realizing they were not for me).
BTW, Hypnotico, I think you're one of the most attractive of our young members here, with an irresistible smile. Maybe that's why some rather insensitive guys ask you such a personal question, it being inconceivable to them that you wouldn't be hitched already. It may be their idea of a compliment.
Answer that you enjoy dating at this point, but really too busy with school to do much more for a while yet and before seeing where you might relocate (actually a wise strategy for yourself), but after getting yourself settled you don't expect to stay single for long. All true, correct? And if the guy doesn't like hearing that mature plan for your life, and can't see the logic of it, maybe the problem is with him and not you. He's an example of Doug's jigsaw puzzle above.