Gaining

Ooh the classic dilemma. I feel you man, had the same thing, was an athlete with these toughts. I first tried convincing myself i was only a feeder and wanted women fat but eventually gave in and now just got chubby. Is it bad, no. So you can try a bit and see if you like it? I can still do sports with an extra 15kg (yes yes im slower). If you really want to stay as fit as possible try roleplay or just feeding others as a start.
You can always lose again

100% had this issue & I fought my Feedee urges most of my life… got a degree in health, wellness & fitness, was a personal trainer & group fitness instructor & was in the army reserves so I had to maintain my weight & physical fitness. I started at 145 out of college, gained up to 185, lost weight & lifted a ton & got to a lean 165, then gained up to 240 & then dieted back down to 185. Once I got out of the reserves & was happily married, I stayed at 200-210 for a bit until I gained back up to 240 again. It’s hard repressing a part of yourself for so long, it’s exhausting. On that note, I do still believe health is important & there are ways to reduce the risks of being bigger (exercise, sleeping well & enough, having a balanced diet) there is still more health risks being bigger vs thinner, but being stressed from suppressing a part of yourself is also not healthy (did it for years) so pick your poison & don’t care about other peoples opinions.

Ditto here. I try my best to not have binary thinking on the topic because it’s when I get in unproductive thought spirals and find myself unsatisfied whether I’m thin or chubby. Allowing my body to fluctuate gives me the best of both worlds. Role playing is also great.

While there are valid reasons to want to be slim or visibly muscular, it's worth pointing out that recent studies show exercise, compared to keeping a low weight, is more important for long term health.

education.virginia.edu/news-stories/why-weight-researchers-say-its-fitness-matters

Of course this isn't to say carrying extra weight isn't hard on your body in certain ways, and overeating can cause gastrointestinal distress, but I think dispelling the "slim = healthy" myth is worthwhile. People who are slim but don't exercise are often more at risk for cardiovascular disease than people who are overweight or even obese who do exercise.