I am going to deny my boyfriend when he makes sexual advances
Why would you deny your boyfriend's sexual advances?
He's feels like I don't give him enough sex.
If you feel that you are not enough for your boyfriend and you cannot satisfy him, then you should break up with him. It is not appropriate to deny him sex all the time, it is best to discontinue the relationship altogether. This is a more honest response.
Yeah, but I don't like being honest.
You need to be honest with him. Staying in his relationship will do neither you or he any good.