If a guy wants to have anal sex with his girlfriend this doesn't make him gay.
What makes a person gay is wanting to have romantic and sexual relationships with other guys, not wanting to engage in a particular kind of sex with someone of the opposite sex.
Anal sex is often associated with gay men, but remember, everyone, gay or straight, male or female has an anus!
And of course, plenty of gay guys don't have anal sex at all with their partners. As one reader said in the comments:
"Well, my boyfriend and I have been together for a little over a year now and we’ve had anal sex about 4 times. I can honestly say that the IDEA of anal sex was much more appealing then the actual process. Anal play we enjoy as well as MM [mutual masturbation] and oral, but yeah actual anal sex/penetration just isn’t for us."
We still don't know exactly what makes a person gay, straight, lesbian or bisexual, but we do know that it's not a desire to have anal sex!