A lot of these replies seem to involve the wife not being keen or responsive despite the husband trying or initiating sex. Or a breakdown in communication eg. no longer talking or living in separate rooms.
My situations different, I could probably have sex every night of the week if I wanted with someone I get on really well with. Problem is I just don't think I'm that sexually attracted to her anymore so don't want to have sex.
My situation was maybe more similar to yours - after 7 or 8 years of marriage the missus started piling on the pounds - to be fair I did a bit as well, too much of the good life, not enough exercise all that crap, but I just stopped finding her remotely sexually attractive. At size 16 she was voluptuous as I've alywas preferred curvy women with great boobs and a grippable arse - but at size 20 she just didn't do it for me at all. She probably would have been up for more sex but even though I was often horny, it wasn't for her.
I was miserable frankly - tried all sorts of diets together and tried to encourage her to exercise, eventually I lost 5 stone in weight and got seriously fit and she was always like "you should love me for who I am". I did love her for who she was, but loving someone is not the same as spending the rest of your life with someone. I love lots of people I don't shag, wouldn't dream of marrying them.
Don't get me wrong it wasn't just about the weight - there were other issues - but the sex was probably the biggest one. We went to marriage counselling for about 2 years on and off and it never helped because she was happy with her life the way it was, I wasn't and never the twain would meet.
Somewhere in there I also had cancer, nothing major, but really made me think that life is too short to waste a day of it doing something that makes you miserable.
We'd never had kids (no surprise given the lack of sex) which frankly was a blessing as it made splitting much easier. Don't get me wrong it was still the hardest decision I've ever made but also the best decision. All ended about as amicably as could be - no affairs, no punting, just agreed that we were not going to get back to where we were so best to move on while we're both still young enough to find someone else. I bought her out of the family home which makes a great bachelor pad and she bought a nice semi-detached place in the countryside.
She's now got a new guy just moved in with her and is happy as larry.
I had 6 months of amazing punting, ticking items off my sexual bucket list - followed by 6 months with a very hot much younger girlfriend - in the last year I've had without question the best sex of my life and now I'm back punting again I couldn't be happier - so frankly by comparison larry is looking positively morose.
Obviously it all gets trickier if you have kids - I have two best friends who had problems, tried to stay together "for the kids" and it made them even more miserable than I was. They ended up playing away as an outlet for the stress and getting caught - and as a result got totally taken to the cleaners in the divorce. In one case he doesn't see the wife anymore and the kids won't even talk to their dad which is tragic.