I think Karma Coma has a right to decline sex with a man for any reason, at any stage of the game. Rejecting him for penis size is no different from rejecting him for any other reason. As a man, I believe I have the right to look at, smell and taste a woman's genitals, in that order, before having sex with her. If she fails the sight-test, smell-test or taste-test, I have the right to change my mind.
So heres my opinion on the size thing, if yer' big enough to knock her Fallopian tube around like a punching bag, you can make her toe's curl. But thats just my 2 Male cents on the topic.
Size can matter but it is not a rule. I have a very small penis but have had a very prolific sex life.