Im curious

My boyfriend and I have been together for over 5, going on 6 years. In the beginning we were intimate all the time. About 2 years ago, things started slow down and we went from having sex a few times a week to a few times a month. Now we have sex maybe once a month and thats only if I initiate it. I've considered that maybe he was cheating on me but I havent found anything or seen anything that would indicate he was doing so. It seems like im more interested in having sex than he is. Im just curious if I am the only one with this problem?