Most people in long term relationships will gain weight because it’s a normal human thing to happen to bodies.

I'm 32, and I weigh 5 pounds more than I did when I was 22. According to my bodyfat percentage, the extra 5 pounds is from muscle rather than fat. There is some 'natural' weight gain, but it's not much at all. Weight doesn't just happen, it comes from somewhere. Anyone who claims they gain or retain weight but don't eat should contact NASA immediately - they'd be a miracle astronaut because we wouldn't need to spend anything on their food. NASA can promptly inform them that the topic has been studied extensively, and no human is exempt from the fundamental mammalian metabolism that has existed for millions of years.