Why is sexual desire irrational?

Because I didn't put thought into it and decide to have sexual desire. It just happened.

This is wrong. Rationality isn't about defeating emotion. You can't think up your values from a philosophy of perfect emptiness. Terminal values is one of the things that depend on where you are in the mind design space. No matter how your values came into being, even as they are, gorged up by the mindless god of evolution, you use them to decide what to make of yourself, what to expand to the future.