What advice would you give to a guy who doesn’t really know what he’d want to do with a girlfriend, but still feels like a loser for not having one?

lots of good advice here. I agree with the sentiment that having a SO won't make you feel like less of a loser.

But here is my two cents. I think the point of being in a romantic relationship with someone is to hang out with someone that you like and who likes you (and, you know, have sex if you're into that). Really not much more to it than that.