As a girl myself, I'm fine with the romance between Bruce and Natasha as long as it adds to their characters. When romance plotlines are done well, more people view them as character-building. If the romance is just out-of-the-blue with no development or explanation, then yeah, that's a problem.
If there's no explanation on the coupling, then it can easily be interpreted as "Natasha is in love with Bruce because he's the only thing that scares her, because him turning into the Hulk is the only situation she can't talk her way out of, so he's ~*different from the rest*~." If THAT'S the case, then I'm going to have some big problems with this couple, because that's pretty lame.