It's not just that he's creepy. It's that the way he's treated is completely inconsistent with the way he is. Is him hitting on his girlfriend's other lover best friend the second he meets her skeezy? Yes, but not terribly so in and of itself, it's true. What pushes it over the edge is that Kory is turned on. Is the fact that his friends are immature, skeevy drooling fratboy jackasses who spend all their time talking about how HAWT his fiancee is, and Terry laughs and brags instead of telling them to shut up, skeevy? Yes, but not terribly so in and of itself, it's true. What pushes it over the edge is that the Titans are amused by this, when they bother to notice at all. Is him being ten years older than her and her still a teenager skeevy? Yes, but not terribly so in and of itself, it's true. What pushes it over the edge is that the writing emphasizes the difference between Terry's social circle and Donna's at every opportunity, and not one person seems even a little concerned.
That's all a bit wordy, though, so "Terry Long is creepy" becomes a convenient shorthand.