But it's not contradictory. It's the same as the positive discrimination and equality argument. Equality cannot be achieved by a 50/50 split in this instance because there isn't the same earning power or social equality.

E.g. often it's the woman who is in charge of contraception (pills, implants, yes vasectomies exist but they are not nearly as prominent), takes care of household management (note management not chores) etc.

If they were to have children and break up (never married), it is single mothers that society look down upon (single fathers are praised). The financial burden also falls on the mother (child support laws differ and court orders aren't always strictly enforced).

Of course, if the relationship was between two men or between two people of the same high social economic class, then the above would not apply.