I have a serious issue with people who are insistent on dragging up their significant other's past relationships in the middle of their own. Clearly, these relationships did not work out. Yours is going fairly strong and yet you are still peeved at the mere thought that your sweetheart once looked at someone else the way they look at you.
To me, this suggests that you have a problem with jealousy. As if you had never had other lovers before as if you had not taken someone else to prom. As if you had never had a crush on anyone else ever before. I mean, by this same standard, you shouldn't be allowed to have a celebrity crush, either, right? Which is just downright silly.
You are the one who's by their side now, so why should it matter who they were with in the past?
Unless your current partner has a history of infidelity, abuse, and/or violence (which you should always know about BEFORE dating them), there is no reason why you should be so concerned with who they loved or even just messed around with before you came along.
This is especially important if you didn't know your partner even existed when they were engaging in these relationships. You don't get to stick yourself in the middle of ancient history that doesn't belong to you. It isn't your business to meddle in someone's past personal choices and decisions. So you met your beau in college and found out that they—unsurprisingly— went to their senior prom with someone else. So what? Didn't you?
You have every right to be a little bit bitter about having missed out on some milestones you both could've shared. But to get so bent-out-of-shape in regards to not having been their date to your high school prom is rather extreme.
You're causing emotional harm that simply isn't worth it to you or your partner.
Yes, maybe your high school experience would have been exponentially better if you had gone through it all with your current boyfriend or girlfriend. But that wasn't your destiny, and you can't change what's already happened to fit the bill of what you would have preferred. Life isn't always ideal; we meet people at different ages and in different places and we can't help what's already been said and done.
You don't have the right to hate someone just because they got there before you did. You can hate what they DID to make your significant other feel less-than-happy, but you don't get to hate someone they used to date just because they broke up. Just like they weren't right for your sweetheart, your sweetheart wasn't right for them, either. This all can be boiled down to how our society demonizes people who initiate breakups— and a lot of the time, their separations from their lovers don't warrant hatred.
Don't get me wrong, there genuinely are some mean-spirited people out there who can be labeled as such because they manipulate, deceive, or overall disrespect others. Maybe one of your S.O.'s past loves was this kind of person - it sucks, but your S.O. got through it or is in the process of getting through it, and you need to focus less on yourself and more on helping them through their feelings.
Concentrate on the happiness that you two have, not on the happiness you could have had at a time before you had the opportunity to connect.