Another complication / nuance is that, of course you should be serving and supporting your partner (and vice versa)! Its part of what makes relationships rewarding. They're not always 50/50 in all aspects.

The trick, as you say, is to know when that is crossing into something unfair. When it goes beyond something like who does the dishes or makes the most money into supporting the other person's core identity. Or when it becomes unsustainable / exhausting for other person. Identifying these issues can be difficult. It requires both partners to be in touch with their feelings and able to communicate openly.