As a now grown woman who is married to another woman with two children, I'll say this: you may be surprised at your teen's sexual desires and ability to articulate them. You should also tread carefully with invading her privacy. You say that you accidentally came across these texts - seems like quite a few texts to "accidentally" read (but who knows....maybe they popped up on another device that's connected to her phone and she wasn't aware). Back in the age that predated everyone having a cell phone, a girlfriend of mine and I kept a notebook. In it we recorded little love notes and other things that my parents wouldn't approve of. My mom found the notebook and she and my dad read through it. I didn't find out until years later that they had violated my privacy, and it still really stings. Protect your daughter, by all means, but be careful in how you move forward and consider how it will affect your relationship with her.
All of that said, I'd check out PFLAG and SMYAL.
http://pflagdc.org/
https://www.smyal.org/