No, a partner “ letting themselves go” doesn’t justify cheating.
But, I’ll be honest and admit that it can cause relationship tensions when it’s not caused by a medical condition and the partner refuses to address it.
My DH has good hygiene, has no medical conditions ( he has an annual physical), has always had good MH, and is about two stone overweight. His doctor has advised him to lose the extra weight, but he isn’t. He likes snacking and buys himself large bags of crisps, chips ( as in chips and salsa), pretzels, etc.
I do sometimes feel resentful, because i can’t understand why he doesn’t try harder! No one else in our household is overweight, because we don’t eat those bloomin’ snacks! If he cut down on them, I think a stone would fall off him tbh.
It also bugs me as I have an underactive thyroid and had two large babies, yet I work to maintain a healthy weight. Not as slim as I was 20 years ago, but still in the healthy range.
But, I’m not going to cheat on the daft bugger, he has to decide to address his weight and get rid of his middle-aged tummy.