I think that you SHOULD speak to the girl's parents. You don't have to be confrontational, etc. but you could introduce yourself and just say that you know the kids are texting and you want to make sure the content is appropriate. Don't accuse their daughter of anything - just let them know that YOU will be monitoring things from your son's end, and it sounds like they will do the same.
Your son must like this girl or at least feel comfortable talking with her about things, which is a GOOD thing. You don't want HER parents to think he is leading her into bad behavior or that you are not an involved, concerned parent.
I don't think taking the phone away at certain times, etc. will really help. Kids can text each other just as easily from the mall or after school as they can in the evening.
For your son to be talking about sex, condoms, etc. is one thing- doing is another. I agree that you need to have a serious sit-down with your son about 'sexting' and all the really bad consequences it can have for kids. Find some examples on the internet to show him, so he doesn't think you are just over-reacting.
Above all, don't accuse him of anything or freak out on him. You WANT him to confide in you and he won't do that if he thinks you are angry with him or will forbid him to talk to this girl, etc. Let him know clear boundaries for what you think is appropriate and what isn't and talk to him about safety, and reputation- both his and the girl's. Point out that if he really likes this girl or is her friend, he would never want to put her into a situation that could have really embarrassing or bad consequences for both of them. Cast him in the role of being a good boyfriend who wants to respect and look out for the girl he likes.
I think you're a good mom to be dealing with this and not ignoring it like so many parents do! Just keep everyone's lines of communication open and it will work out! Good Luck!