I don't see it either. Then again, I am only a little bit of a Marvel fan, and not a fan of Cap at all, so I might be biased. But still, they just don't click, to me. I don't seek out romance fic, but I'll read Steve/Tony if it's part of a larger or amusing story line.

Plus, I grew up shipping the Trinity: Batman/Superman/Wonder Woman (and any combination of the three). Unlike Cap and Tony, at least in my opinion, they actually have loads and loads of chemistry.