"If she had sex after the alleged rape, would that be evidence that maybe he didn't rape her?"
No.
Really? I don't know what you have heard, but from my understanding, rape is a traumatic experience. Many women cannot engage in sexual activity for many months after an instance of rape.
But, I guess you could be right. I guess she could have been brutally sodomized and later decided that having sex before heading to the hospital to have her examination to charge her attacker with rape was a good idea.