Halloran dislikes Victor. He will always dislike Victor. This is unfortunate, but true, and it is because the heart does not work logically.
He isn't quite okay about Capri having sex with Kieran - her first lover - but he talks himself into being okay with it by reminding himself that Kieran's been her best friend (besides him) since she was born. And Halloran does trust Kieran not to hurt Capri. (Well, not to hurt her any more than she wants!)
But when she had sex with Victor.... She hasn't known Victor very well. He's more of an acquaintance. And Victor's a bit of a slut. So there is a bit of Halloran feeling that Victor is beneath Capri, and a bit of just...
Halloran is, at this point, still a few years away from admitting that he's in love with Capri.
But it's obvious. To everyone but him and Capri.
So being with Kieran is okay, because he's her best friend. But being with this random guy...
Halloran doesn't deal well with it. And he manages to adjust himself to Capri playing around, eventually. But he never really forgives Victor for being the first to "rub it in".