Sex once a month or less! Normal?

Me and my fiancé have been together for 9 years. I love him so much and can't picture myself without him. 6 years ago when I started to gain weight we stopped having sex. He would use different excuses like 'feeling uncomfortable because of his rents being around' or not finding me too attractive. I asked him why he proposed and he said he loves me and always will, but when I try to tell him that it's weird we have no sex life he avoids my questions and pretends like everything is fine. Should I be worried? Why is he avoiding it? Is there anyone out there with a relationship like mine? Thanks :)