Trust me, I get it. The same situation happened to me. My ex was texting to a very flirty girl that he worked with. And while he didn't do anything wrong, he didn't put a stop to it either, which hurt. We ended up getting divorced because I couldn't deal with him having female friends like that. 

You're still young. Nip that jealousy in the bud before you grow into it and it's even harder to get rid of.