Why does he care who I slept with?

So my ex and I have been dating and thinking about giving a relationship another shot. We where broken up for about a year. One of the things he said he needed to know was if I slept with anyone since I dated him or not?

Why does this matter?? It’s not like I cheated on him, I didn’t sleep with anyone over the last year but I don’t see how it matters if I wasn’t with him?