Having dated a woman who had a mastechtomy (only the one) and not being privvy to it until the last minute, as it were, I have to say it didn't make much difference. It took a bit of getting used to at first, but it was no big deal.

The one thing that could have thrown a spanner in the works was that she never mentioned it, until it was literally in your face, but in a way thats a good thing. I can't think of many better ways to test if a guy is serious or not.