Okay, this may seem obvious. They're selling a birthday card.
But what is the woman who buys this card selling?
It doesn't help to know that the inside is a cheap joke about calling Chicken Delight because they have the best legs around. Ha ha.
But step back and consider the larger message. It's suggestive and slutty on the outside and cold-hearted denial on the inside. That's pretty much the exact opposite of what a man wants in a relationship. And, for an added bonus, she's mocking you for wanting what you want.
Why would that be appealing? Why would you want to get into or stay in a relationship with a woman who is sending you that message?