Sounds like a loss of attraction, it happens in many relationships. Maybe your body has changed more than you realize while you've been together and he isn't as attracted to you as he once was but he doesn't wanna come out and say it and hurt your feelings even though he doesn't realize that his actions are more hurtful that simply asking you to go to the gym with him or something. Of course, when many guys get in the mood attraction goes out the window. They just wanna get their rocks off which it seems is the case with your guy.

People always say love should be unconditional but our personal physical attraction isn't necessarily unconditional and that can't be helped. It hurts a hell of a lot when it comes up in conversation it can end relationships so its rarely if ever brought up and many guys just learn to live with their lady gaining weight.