Well, that actually why I think it makes sense with Garfield. Because Terra was a bastard (literally and figuratively) even when she was pretending to be nice, especially to Beastboy because he was always in her face. And he still kept going at her... and unlike the cartoons, she didn't have a last moment change of heart because of him.
Garfield, if he was in denial, leapt at Tara because he figured that if no one else would get close to her, maybe he might and no one would suspect the truth.
Like, not that long ago I read about a woman who claimed she had sex with a hundred men in order to convince herself she wasn't gay... which I think it a state many gays live in because of the stigmata that coming out would bring.
Actually, this sounds like something that could be a decent story. Like imagine Deathstroke, with his hate-on for the Titans, finds out and uses it to blackmail Beastboy into being his informant on the Titans? Garfield is caught between protecting himself and the image he thinks he has, and his friends. It goes back and fourth inside of him every-time Slade gets a report. And in the end they have this big confrontation with Deathstroke and Beastboy where Slade again tries to push Garfield into turning on the others... only for the team to reveal they already knew, but also knew he wasn't ready to tell them and were just waiting for him to do so. Slade's control gone, Beastboy kicks his ass and everyone goes home happy... and Nightwing tells Beastboy to stop looking at his butt