Funny thing...you wonder how you can love her so much and other people just use her and if she allows that, she must be a slut.
Here is the good news: date her for a while and you won't care. You will learn to hate and resent her for many other reasons--other than being a slut. Jk...kinda.
It's just the new girl blues. As you get older, everyone has a past. She will eventually be yours--unless you are foolish enough to get married that young. By the time you get into your 30's if you aren't in the high teens or low twenties (sexual partners), you look like a damn prude.
As I see it you have two problems...very small problems albeit, but here goes:
1) you ask the big 3--Am I the best? Am I the biggest? How many partners? The answer to those questions either suck, or they are a lie. Assume that.
2) you let some chump come up to your girl at a party and ask if she missed another guy. Interrupt that conversation and introduce yourself. I am not saying fight, but show that there is a sentinel at the wall standing guard if Allan gets the balls to do his own bidding. Hell, your girl will respect you more, and you won't appear weak. Plus, you don't have to kick yourself for the entire car ride home. I would rather have someone else kick my ass than for me to do it for them...against them, you always have a puncher's chance.