Can buy it as internalized fatphobia to show they don't have to "settle" for fellow fatties or defy societies ideas that the only people they can date is fellow fat people. Some fat women may prefer to get it on with skinny dudes or hunks and to show that their only dating pool isn't only limited to fellow fat people. But this isn't universal, I'm sure there's fat women who fantasize about being with fat men and vice versa.
Cousin did mention times of fat men having this internalized fatphobia. Where some really fat dudes mocked small fat women. Like meaty hourglass level fat, and saying they're super obese and undatable when they're way fatter than the women they bully. If they internalize the double standard that fat men can exist, but women being fat is a horrible sin. That women can't put on weight or be fat, and we must treat fat women being nude/sexual as gross and disgusting. As well as good ol' hypocrisy. Like the scene in family guy when Lois gains weight and she says peter is fatter than she is, and he responds "Lois, fat men aren't fat, fat women are fat".