Why Does My Boyfriend Stick Up For Other Girls?

A lot of men do that. Guys are usually raised with the knowledge to always treat woman with respect and will help them out when they feel a girl needs it. It is usually a sign that you have a good guy that respects woman.
If he is sticking up for other girls and not you then you should have a talk with him about were you stand in the relationship and if you should even move forward. If he's not there for you then you should maybe not even be there at all.

First thing- Is he sticking up for girls that YOU have put down? If so, stop doing it. Guys don't like to listen to their girl say bad things about others. If he defends them, he's trying to tell you something. If you keep doing it, you make the girl look better and you look worse...and vice versa too.  If he's defending them against others, then hey, he's probably just a nice guy and the same rule applies to the person on the offense.