> Am I forced to basically change positions or practically do a push up?
Yes.
At least one partner not being outrageously unfit is very helpful in avoiding bad sex, especially when at least one of them is fat. If you want to have good sex with fat women then you should go to the gym.
Cowgirl is also more of a fitness thing than a weight thing. I've had sex with women who ride very well at 350+, and women who get out of breath at ~225. Though either way, the partner on the bottom does not need to be passive - you can do anything from help out, to pretty much doing all the work for her. Sex furniture can also be helpful, e.g. https://ridethecowgirl.com/products/riding-sex-seat