To me, it depends on whether or not the weight gain negatively affects the physical aspect of sex.
When I met my wife I was 190 lbs, she was like 110-115, both of us young and pretty sexy. We then both worked at a callcentre for several years, living the usual shitty-shift-work life of eating nothing but fast food, hanging out with friends, playing video games, and whatnot. She gained maybe 30 lbs total, and I ballooned to 300 lbs.
Fast forward several years, some stress, the usual life stuff, and both of us changing careers to something we both enjoy WAY more, and we've both managed to lose some weight. I don't we'll ever be like we were 13 years ago, but I've dropped to around 250 and feel vastly better, and she's sort of hovering around a reasonable weight as well.
We're both still overweight, yes, but here's the thing: since our life has gotten better overall, financially and otherwise, we've had more time to focus on living well. Not necessarily hardcore exercise and dieting, but generally eating better, and having fun with things like hiking, walks, kayaking, that sort of thing... And we eat better now too, making almost all of our own meals from scratch, no frozen or processed crap, and very little fast food.
Our QUALITY of life is better. And you know what? Our sex life is MASSIVELY better now, we have sex way more often, and the sex itself is way better. DESPITE the fact that we're both older (early 30s now) and overweight. We're both comfortable with each other's bodies and the extra flab doesn't bother us.
My point being, if your weight gain is making sex PHYSICALLY very difficult, then yes, there's a big problem and you have to try and fix it. If your weight gain is mostly an APPEARANCE problem, it really shouldn't affect your sex life--if it does, then I suspect it's an overall problem with the relationship or your current quality of life.
That's just my opinion, from the male side.