It is him, it isn't you gaining a few pounds.

My first husband liked to look at porn and masturbate and he and I seldom had sex. I was angry at him for choosing the porn over me but it was his choice so there was no way to fix it. After being married for seven years we divorced over other issues. My next BF liked porn but watched it with me and it was a prelude to our sex which was great.

I don't know how you can solve your problem but don't blame yourself. I'd wait a bit before I'd marry that guy if I were you. He might not be the best match for you.

Just my opinon...