A: Just because your boyfriend is interested in being anally penetrated, that doesn't mean he's gay, or even bisexual necessarily. Many people associate anal sex with gay men, but a lot of straight men also enjoy anal stimulation and taking it in the booty.
Men actually have more of a reason to enjoy receiving anal sex than women, because of the position and sensitivity of the prostate gland (sometimes called the male G-spot). Stimulation of the prostate can produce more intense orgasms and ejaculations, which is why so many men like putting things up their butts, especially while masturbating.
You didn't know that? Well, it's true. Guys just don't talk about it and, well, it's not really something most men would readily admit to. For the more liberated fellows, there are a number of insertable toys that are designed especially for prostate stimulation, such as the Aneros line of products.
But back to your boyfriend. Don't worry - boinking him in the butt won't turn him gay. Actually it will probably make your relationship closer. Anal sex is a very intimate act; the person being penetrated is in a very vulnerable position. The fact that he asked for this shows he trusts you, not just in opening his backdoor for you, but just by being open about his desires.
If you decide to go ahead and give your boyfriend what he's asking for, my advice is to start out small and go easy on him at first. Get a harness and a dildo that's no more than an inch in diameter. These are often sold with a butt plug and other toys in "beginners' sets". You might want to get a book on anal sex - like Anal Pleasure and Health by Jack Morin - and maybe check out the "Bend Over Boyfriend" series of videos.
Also, don't forget to get some lube. You'll need it!