Ok, this whole situation sounds hard for a parent but there are a few things that need to be covered.

You gained custody 3 months ago, what was your relationship like before? Could she come to you for advice or has she always been a bit secretive and distant with things like boys and sexuality in general? What was her home life like before you got custody? How does her bio mother feel about the whole situation (if she knows)?

I'm not judging you, at all.

I know what it's like to be an awkward teenage girl, and talking to your dad about boys...'like OMG...totes gross'

I'm just thinking of your lil girl, how did she met such an older guy?

From what you've said the messages haven't been sexual so I don't think you need to worry about her being coerced into anything dangerous or provocative...yet.

However it might be worth having a relaxed conversation with her about guys before things progress.

You know. How teenage boys literally only think with their cock and only want one thing.

Don't make it a lecture, try to be funny and honest about the actual act.

You will die a little bit inside as you talk about it, but the more you humanize sex the more she with understand that it's nothing to getted worked up over and an ACTUAL relationship is more important.

It'll be really, REALLY awkward but she needs to know and I think it will help you two to connect.