How can some younger people be so intrusive?

Had this pop into my mind now so I wanted to write it down (Yes I know it sounds like I'm an old man complaining but its true.) Had this co-worker of mine while I was working just observing me I guess, and I'm a quiet, reserved, to-myself type of person and as a couple days goes by (we've has conversations about video games, music and other stuff before...small chitchat) he asks me if i have a girlfriend, i say no, shortly after i asked same and he said he had one, then a little bit after he asks me if I'm gay or something...i answered does it matter???

This literal fetus came to the conclusion i was gay from me being reserved, doing my work, not really being super vibe-town miggie with him, and saying I don't have a girlfriend...yes I am gay...but my problem is when people come to f*cking conclusions with people that just arnt really up to vibing with you and talking to you about all this stuff. He would've came up with the same conclusion with someone who is pretty much the same, reserved, working, not really being super talky talky and it wouldn't have been true at all.

Mind your own f*cking business. It's not hard thing to do.