Why doesn’t he want sex ?

So we’ve been together for 3 years now ,

And over time the amount of times we have sex just keeps going down and down ....

I keep asking him what’s wrong and he says it’s nothing I absolutely don’t think he’s cheating on me so please don’t say that .

Is this normal ? This is the longest relationship I’ve had and I’m worried maybe I’m just over reacting . I know he really loved me . Is it normal for sex to decrease we use to have sex multiple times a week and now it’s like once every two months ..

He told me he sees something deeper with me and that’s why he doesn’t care about sex

Btw were both 24 , aren’t 24 year old guys suppose to be horny all the time ?