Fatness ≠ state of health
This whole "I don't date fat people because they're unhealthy" is tired and annoying.
I've dated people who are wayyy bigger and also wayyy skinnier than me... the skinnier ones were all unhealthier than the biggest guys I've been with in terms of diet and lifestyle.
It's wild to me that skinny people who smoke, drink, not work out at all, eat utter junk, have bad sleep hygiene etc have the audacity to think they're healthier than fat people because of size alone. As if taking care of what they consume, how active they are, keeping their mental balance in check etc doesn't apply to people in the 'average' range in BMI (which is outdated, btw).
Let's be so real right now. Not finding fat people desirable because you don't like how they look doesn't mean you actually know how healthy they are. You cannot tell someone's overall state of health with the naked eye, you can only make assumptions.
If you're shallow and don't want to unpack your internalised prejudices, just say that.