Read other letters about this article
Not in every case, likely not in this one, but it does. Be honest with yourselves, men and women both: if you started dating someone, and a fair way into the relationship, they told you they'd cheated on every single one of their previous partners, would you assume you're the one who'll break the pattern, or would you do the sensible thing and bail? Being judgmental in a relationship, in the sense of forming judgments about other people, is inevitable. It's not only inevitable, it's healthy. Sometimes, actions speak louder than words, and somebody telling you how they've changed and things are different now is just the first step down the primrose path to hell. If you've slutted it up, (some) people are going to judge you harshly for it. Deal. If you're so sexually liberated and indifferent to Puritannical societal norms, you ought to be able to put up with a little opprobrium.
A bit of youthful sexual adventurism, like this, is a different animal. It doesn't necessarily presage future activity. But if the BF has a problem with it, that's ok. He needs to be a mature adult and end the relationship. But this situation, as with most human interactions, is complicated and has more than one side. It literally blows my mind to see people patting themselves on the back on how tolerant, open-minded, fair, etc. they are, then going on in the next paragraph to denounce someone they've never met and have only a few slim words of description about as a hypocritical, possessive, abusive, vile, mean-spirited, knuckle-dragging brute. He's not acting wonderfully, but people make mistakes - maybe he deserves a little of your liberal compassion too.