Great question! I don't have personal experience with this one, but have had a number of close friends who are gay. I think every one of them would agree with me when I say, Handle it the same way you would if it was a boy - almost.
First, from the way you've worded the question, it seems you don't believe she should be having a highly sexual discourse (or relationship) with anyone at her age. You'll need to mostly focus on that fact with her. She'll want to focus on the fact that you were snooping as a way to avoid the rest of the conversation so be prepared for that fact. In my opinion, if there isn't already a prior understanding between you about it, own up to the fact that you snooped, apologize for it (because it is a break in trust - even if you feel it is a justified break) and then insist on moving on in the conversation to discuss the sexual content of their texts.
It may be a joke between the two girls (teen-aged kids have a strange sense of humor) so you still shouldn't jump to any conclusions, but I would approach her and say pretty much what you'd say if it were a boy on the other end of the conversation. The only difference is, you'll have to clarify for her that your concern is about the sexual content and NOT the gender of the person on the other end of the conversation.
If she says it was all a joke, remind her that you would still love her if she were gay (in case she is just covering - and because telling them you love them no matter is always a good thing anyway - especially when you are mad or concerned) and it isn't okay to have highly sexual telephone convos even when it is a joke.
Make sure within the conversation to offer her room to speak - try to keep it a conversation and not a lecture and you will get a lot further with her, she is more likely to open up more with you and you'll come out of it with more knowledge about your daughter and hopefully a sense of trust.
This question and its answers may offer up some helpful thoughts and does contain some discussion about the "snooping" aspect of things, it is a question posted by a parent upset to discover her child might be gay - so is quite different, but some of the ANSWERS are closely related.