Yeah, a guy liking anal sex alone doesn't make him gay, I think. It's just a different way of pleasing him. To help prove Mittimer's point some, I consider myself to be straight, and I like giving myself anal with a dildo I bought a while ago. It just...feels good! Sometimes it'll stimulate the prostrate, which helps make it feel good.
It doesn't sound like BDSM to me, just the opposite of what natural intercourse is. Instead of the guy putting his penis in the girl, the girl puts a penis in the guy, so I don't see how one is BDSM without the other being as well. Don't worry.
I'm glad your husband finally came out with it that he likes it, hopefully over time he'll be more comfortable with it and not worry so much. Also, it may be considered out of the ordinary(or it may not, I'm not sure unfortunately), but if your friends end up not understanding that your husband likes anal(and think he is gay because of it), then that's their problem. You and him both know he's not gay, and that's what should be important.