Someone either being exclusively a top or exclusively a bottom is probably the exception rather than the rule. Also, just as with het sex, just because one person is being penetrated does not mean they're being submissive. A "bottom" can even be on top and controlling things, i.e. "riding" it while the "top" is lying on his back relatively passive. Not all gay people even like anal at all. Some just stick to other forms of sex like oral.
I admit that I'm kind of into dom/sub play, but that's just me, and it's not crucial at all. Just something I can get into if my partner is also into it. Regardless, it's very important to me that my partner enjoys sex. They're not going to come back for more if they don't, right?