Surely, you are thinking of a powerful charm spell? Because even if they were gay, it doesn't mean they would fall in love with you.
Anyway, I thought everyone was bi, DAII-style?
I'm sure it'll be possible with a charm spell, yeah. If not, I doubt it'll take much tweaking in the creation kit to come up with a suitable mod for the job.
EDIT: I'm just glad there's romance this time around. So many of the Oblivion companions were difficult to feel an immersive connection with. Probably because they were always bumping into stuff and grinning stupidly at you. It was like dating a mentally slow puppy dog.