Can I say it depends on his partner? If the woman wants greek, it doesn't change the fact that he's straight. He's just plugging a hole. If he likes guys too, then he would be bi. He doesn't become bi or gay until his wants or intention is focused on the same sex.
Just because a woman enjoys oral performed on her more than intercourse, doesn't make her a lesbian. She might get more satisfaction from oral, or maybe the guy she's with can't give her an orgasm while having intercourse.