If you don't know, this is called sexting. It is common an not okay with me. I do read my sons texts, but not often. You only have to do it once to get the point across, what was worse for my son, was that once I read something that I didn't like, I discussed it with him, and used it to open a complete sex dialogue, I haven't seen anything like that since.
About his privacy, the point is to raise a safe secure stable individual, not to be his friend. This is your job. He may secretly be happy about it, because he may be very uncomfortalbe with it but not know how to deal with it. Now he can say, "my mom reads my texts, so dont' send me that stuff".
I think you have a couple of options:
1) Bare minimum is to have a talk with your son about why he thinks that these girls are doing this. Maybe they are insecure and feel that this will get them attention and get the boys to like them. Ask him how the sext's make him feel and what his ideas are to deal with it, because if they don't stop then you will cancel the texting on his phone.
1) text back to the girls, let them know who you are, and start a sex discussion with them about respecting their bodies, etc... they will get the point.
2) call the girls from your sons phone, let them know who you are, and ask them if they understand why this is not okay, etc, and offer to meet with them with their parents for an open discussion, with them, their parents, your son, and you.
3) call the parents and let them know what is going on... or better yet forward the text messages to the parents cell phones, without any comments from you, the parents will get the point, and you won't be the one they blame. because there is always that chance, when you "criticize" someone elses child then you are "criticizing" the parent, even if you are trying to. These are their kids remember.
Bottom line, figure out your objectives, to get them to stop or to help your son, or to help the girls, and then do what is necessary to meet the objectives, nothing more, nothing less.