Good question. I feel that the answer is because with men sex is very difficult to get, you need to impress the woman and make want to have sex with you through various techniques and/or flatteries.
With women, it is more waiting for a man to come and offer you enough 'presents' so that you will want to sleep with him.
So it is more that a woman is stagnant in the conquest and the man is actively seeking her, jumping over hurdles so that he can have sex with her if you will.
Following that it would therefore appear that the man is conquesting, while the woman is the spoil of the conquest.
Therefore the man wins the conquest and gets the sex, and due to his victory the woman is obligated to perform.