My Boyfriend Wants to Have Sex With Other Women

We have been dating for three years. Everything is good between, but lately he has been telling me he wants to have sex with other women because he wants to enjoy the rest of his twenties. He has asked me if it was ok if he has sex with someone else. I stupidly said yes because I figured if I didn’t agree he would do it behind my back. Wanting to have sex with someone other than me, makes me feel like I am not enough for him. I don’t feel like he cares about how I feel even after I tell him exactly how I feel about it.