Why does a guy say that sex isn't all they want but constantly talk about it?

If a guy only and ever talks about sex, forsaking other basic needs such as food and shelter, then sex is probably all that this particular man wants (or believe he wants). By "constantly", I presume you mean "often". Which should lead you to believe that this man often wants to have sex.

Another thing to consider is fulfillment. If this man had lots of sex available and no food, you might find that this person would constantly talk about food, though admit that it isn't all that they want (they also want sex!).

By the way, is there a problem? Are you not having sex with this man? You at least don't like hearing about him talking about sex. Maybe you should find a man who talks about sex less, or have sex with this man more so he'll shut up.