IIN that I've never had a boyfriend? I am turning 21 this fall, and I'm only slightly eccentric. I am a modest Christian girl who has never held a guy's hand, and never been kissed. I am thin and rather pretty. I can't figure out why I can't get a decent guy.

That line, right there, is why guys should be falling at your feet. I'll never know why the world is the way it is.
Whenever you'll meet the right guy -you'll know it, until then there's nothing to stress about.
However, if you'd like to have a boyfriend, I recommend getting more social. If there's a guy you like, ask him to hang out after school/work/church/...
Personally, I would suggest you to be yourself and wait for the perfect guy. as you can see many guys would judge you before getting to know you. Just wait for the kind gentleman to show up. But by wait I don't mean literally...go out, meet people, but don't be stupid, meet people you know you can relate to, and eventually you'll find the thing you're looking for.