What's even weirder than him constantly talking with a younger coworker about sex is his need to repeat the conversation to you for no reason at all. Does he want you to feel jealous? It sounds like he is playing some sort of game to me.

In answer to your questions, it's not normal for him to be having these in depth sexual conversations with other women all the time (and repeating them to you). If he's fairly uncommunicative with you except for repeating these conversations, it's natural that you'd focus on them. If he doesn't reply to you but then tells you that he's been spending his time talking with other women -- it's normal to be upset about that as well.

Are you still smitten with him?