I disagree with this. I don’t think MJ is an overly materialistic person, so it’s not like she’d leave him due to wanting more money or anything like that. And in a normal case, I’d agree that Peter’s seeming lack of career ambition/goals and flakey-ness would be perfectly valid ground to want to leave a partner, but MJ knows that Peter obviously has the extenuating circumstances that all those things are caused by him, y’know, being an actual superhero. And if anything, Peter’s dedication to helping others even at his own expense becomes just another positive quality of his.