Just because someone is fat doesn't mean they prefer fat partners. She was what you liked and you were what she liked. Both of you ended up changing things, you a lot more. So, it is just a matter if either of you can accept the other. It's not fair for anyone to try to change someone else.
I disagree that it is easy. For most people, the initial weight loss is the first part of yoyo dieting. The second part which usually comes 2 to 3 years later is the regain. Many times it is more than the initial weight loss, making the person even bigger.
If you don't want to lose weight, tell her so, it is your choice. But if she is no longer turned on by you, or wants a divorce, that is her choice.
13 years
