My wife is fat and I don't want to have anymore?

OK we all get older and things sag a little. I'm not dissing my wife for this it's not her fault. But I'm not physically attracted to her anymore as a result, and I don't want to have with her. I love her of course but I just can't do the thing - the thought of it grosses me out. What am I to do, put a bag over my head? Close my eyes? Suggestions?