I am quite firmly in the "what pleases two consenting adults is their own business and no-one elses, provided it falls within what is legally considered acceptable for two consenting adults to do" camp. We come in all shapes and sizes and all of those shapes and sizes are mere physical variants, so if you get your kicks off having more partner to have fun with, no skin off my nose.
Why shouldnt bigger people be sexual? Do their genitals and emotions suddenly stop working because they gained weight?
No, of course they didnt. And if you go on gay dating sites, which I have in the past with a gay friend who was looking at what was about, there are MANY categories of attribute that people can choose in terms of what they are looking for, whether that be uniforms, piercings, tattoos, other body modifications, skinny, fat, bald, hairy. If there is a variation, there is usually a category to cover it.
Men, gay or straight, often like to be visually stimulated by imagery of their preferred tendencies. Thats just the way it is. Of course there are health risks, and sites like that SHOULD be age restricted to those in adulthood.
Maybe it isnt healthy, and maybe they are taking risks with their own health - but people do that every day, in all sorts of ways. As responsible ADULTS that is their, and our choice. There is nothing illegal about what they are doing as I can see it. Pro-ana (encouraging anorexia/eating disorders/extreme weight loss) is not healthy either and yet sites about it exist. But Anorexia is considered a mental illness - yet overeating and extreme forms of obesity are not in the same way.
There will always be vulnerable people on the internet who are drawn to sites because they want to achieve something that society frowns upon.
But fat or thin, fat lovers, or those who fancy people who are painfully thin... they all have sexual feelings, the same needs and desires to be wanted and desired - and some of them have what would be considered fetish or extreme desires. If it is harming someone vulnerable, that isnt a good thing, but if it isnt, and responsible adults are choosing to do with their bodies what they want - who is anyone else to judge them on that?