How can I not be mad at my friend for talking about guys all the time?

Tell her you don't want to hear her talk about that all the time.
You can say things like.
Can we talk about something else? or Did you see that new movie? Something like that,changing the subject.
If she persists on still talking about it, tell her that you don't want to talk about guys because of how they all treat you. If she becomes irritated or angry with you either leave and talk after she has calm down or just don't be friends with her.
Real friends bend a little for each other even for minor things like this one.

Sounds like you are both a little immature. Why can't you just ask her why she feels the need to constantly talk about her guys? Why not just tell her she has a self-esteem issue since she feels the need to constantly be around guys and at the same time, have your envious loathing bolster that fragile self-esteem? Are you really "best" friends? Why do ALL guys treat you like shit? You must be a very efficient person to have known ALL guys enough in order for them ALL to treat you like shit. I have known a lot of women in my life, and many of them treated me like shit. However, I can never say that I knew ALL women and that they ALL treated me like shit. I mean, there are about 3 Billion women in the world at the moment. Now, if I were Santa Claus, maybe I could visit them all in a day or two.

never mind.

Just tell her to STFU already. If she truly is your "best" friend, she will blow it off and hopefully heed your wishes.