I ask you... is there any proof he isn't?
Wiki shows us no proof, he'll be 60 this year.
...Never been married, No Kids, not even a mention of a girlfriend. No mention of a boifriend.
A 60 yr old man has never had a significant other in his life?
Graham does seem a bit effeminate, but in the absence of any evidence that he's homosexual, it's just as likely that he's basically asexual. Back in the day, some "confirmed bachelors" were closet homos, others just didn't care one way or the other.
Frankly, it doesn't matter that much since Graham's politics are probably closer to Hillary Clinton's than to most Republicans.