This is a misquote from the article. The average weight gain of an adult is under 3 pounds PER YEAR. Not over the entire period from youth to middle years. The average weight gain overall is about 20 pounds.

A change in bone density alone will account for a goodly portion. Also, people who lift weights will add a little extra, both through an increase in bone density and muscle mass.

Gaining 25+ pounds in 1-3 years could be the result of a million things. It could be a medical condition, in which case I would hope divorce is not the first go-to in someone's response. It could be that your partner was bored with you for some reason. Talk it out.

If someone is willfully not taking care of their body, it usually means something else is going on. Talk that out, too.

If the reaction to putting on weight by your partner is ducking out of the marriage, I sort of see a bigger issue here.