NO it is NOT OK, and you better stop "thinking", and find out exactly what his intentions are.
You NEED to know if he called her, she called him, how long they have been talking, and WHY they are talking about SEX. If they were not intimate, what reason do either of them have to talk to each other in the first place. She is doing her best to entice him back to her, and if he has not gone back already, he was seriously thinking about it for sure.
When people lie or play games, the truth always ends up coming out, so if you found out by "snooping", so be it. Women have very strong intuitions, and men underestimate their ability to figure things out. He may try to fault you for "snooping" but do not feel guilty about it - - you needed to know, and he would not have told you, so he created the problem, NOT you.
Confront him. You need to know whether he really is committed to you or not BEFORE you get married.