"Think about this: I go out for my birth day. The restaurant brings out a free dessert for my birthday, and because I'm such an awesome person/just spent a lot of money/whatever, they bring a second for my boyfriend. Do I (a) eat my dessert and let my boyfriend eat the other, or (b) eat my dessert and box the other to take home cause it's my birthday. Of course I (and I assume everyone else) choose A. I think choosing B, shows the person to be selfish and I this could be the only selfish thing the woman does or it could be a symptom of a big problem."
It's even worse than that. Imagine that your scenario happens, but it also happens that you and your boyfriend have the same birthday so the second piece is to celebrate his birthday. That's what makes it so obvious that it's a matter of disrespect.