Even the airline case can be handled with an exception - if "other or unspecified or not asked, assume whatever would be the maximum".
Or just ask people their weights if that’s what you really need to know.
The average American is already way over the "flight weight" - see https://unitedafa.org/news/2021/6/10/heavier-passengers-on-p...
Some planes have weighing systems built into the landing gear but most passenger flights are WAY under max weight (Airbus can tell the pilots if the weight/balance once underway doesn't match what the computer was told).
There might be some exceptions, like an airline won't accept a non-binary gender because of safety (weight distribution estimation). For a dating site it might make sense to allow anything.