Try arguing that a girl being anally assaulted is less damaging to her than a boy receiving oral favors might be difficult to prove clinically.
If there actually was some research that suggests this is the case, I am sure the court would entertain the greater good argument. The, we find it ickier for a boy to get oral sex, than a girl to be anally violated argument really doesn't have much legal weight. This law wasn't really well thought out it appears to me.
They could have made a law against anal sex, let it apply. Considering that most heterosexuals usually do vaginal sex, but still engage in the alternative, it wouldn't be totally discriminating, per se on it's face. Again, I see the Texas problem. Different rules for different receivers of anal sex must be not just punitive by class in nature. There must be a justification, as one could well argue between anal and vaginal sex risk wise.