What would you say if your boyfriend did this?

My boyfriend came over one night, and got a text. When he went to go through his messages, he turned his phone away and then deleted something, and was just acting awkward all around with his phone. So it was quite obvious he was hiding something. After explaining to him that I'm not oblivious and I knew he was hiding something, and a few HOURS of silence, he eventually came out and said that he had been talking to his ex partner. And I know you guys don't know her, but I can personally say she is slutty. Anywho, he said he didn't want me getting mad that he was talking to her. But I don't care that he does, he can talk to whoever he wants. It's not like I would have even seen them because I don't go through his phone. But he hid it, and that makes me upset. And he said she had said some SEXUAL things toward him that he didn't want me to see. Mind you, this is the girl who threw herself at him the first day they met, and got into bed with him at the snap of a finger. And she's texting him sexual things and talking about how she misses all those "times". And he still continues to talk to her. Is this okay? How would you react? It didn't really start to bother me until just now.