When men (and maybe women as well) get aroused they seem less sensitive to "hints" that their partner is not interested. If you don't want sex (or some particular sexual activity), be VERY CLEAR. Slightly fuzzy, "don't really feel like it now", moving away etc may not be clear enough. For the sake of both partners - please, say "No, not now", or something similarly completely clear.

It would be nice if people were sensitive to their partner's wishes, but often they just aren't.