That explanation would still leave open why overweight wouldn't date other overweight people?

Obviously it doesn't go for every overweight person, but I would imagine not taking care of yourself enough to sustain a healthy body weight also comes with a slew of other problems such as depression, low self esteem, social anxiety, that would hinder a healthy sex life.

The behavior patters that lead to obesity usually go hand in hand with avoiding common hookup/dating spots (i.e clubs). Sitting on the couch eating 3 papa john's pizzas every night does not help you bang anybody, regardless of body type.

Only some overweight people fit that description, I certainly don't. You can be reasonably active, go out, and eat pretty sensibly and still have trouble managing your weight.

Still even the more couchpotato types are sexual people and should naturally end up forming dating scenes even if the theory of segregation turned out to be strongly true, excluding people who lose their libido due to depression etc.

(ftr, I see lots of couples with mixed body types but I'm not a millenial or living in the US)

If you are fat then you are not eating sensibly, regardless of your excuses.

For many people, once you keep a certain weight for a while, your body will try pretty hard to keep you there. This means that your metabolism will slow down if you start doing the straightforward small calorie deficit.

Hence to lose weight you need to keep your calorie deficit at high levels and feel significantly hungry for a very long time.

This comes with a risk of yoyo-ing, which is much worse for you than keeping the same weight since it will put high stress on your insulin/lipid metabolism that causes insulin resistance and type II diabetes. Indeed most overweight people have gone through cycles of yoyo-ing and ended up heavier than what they started with.

TL;DR: if you think obese people are just unusually lazy or weak willed, you aren't getting the picture.

The difference between obese and normal weight could be 100 extra calories per day.

Look at all those "healthy snacks" - a small handful of nuts, a smoothie, a pot of fruit. These are all over 100 calories.

Being overweight yourself doesn't change your perception of other overweight people.