It's sex no matter what. But in a BDSM context, I don't think that either giving or receiving anal sex is necessarily submissive OR necessarily dominant.

Let's say I have a hot submissive boyfriend whom I haven't allowed to cum in, say, a week, and I tell him that I'm dying for a good fuck and I want him to do me in the ass -- but he has to wear his cock ring while he does it. Doesn't seem like an act of domination in that case, does it?