I know I already responded to this thread, but 10 months later, I'm just going to add that I almost exclusively think about females (usually with strap-ons!) when my husband is going down on me... and this is totally normal for us. He and I have talked about it, and he feels absolutely no discomfort with this at all, which makes me feel very loved and affirmed.
We both fantasize a lot more when we're going down on each other, for whatever reason... so for me, that's when my bi-curiosity kicks in. I very rarely fantasize about men... just women doing me with strap-ons!
What does that mean? I don't really know, and don't really care... as long as it gets me off.