You're probably thinking about hypergamy.
There is also BRIFFAULT’S LAW:
Where the female can derive no benefit from association with the male, no such association takes place.
1. Past benefit provided by the male does not provide for continued or future association.
2. Any agreement where the male provides a current benefit in return for a promise of future association is null and void as soon as the male has provided the benefit (see corollary 1)
3. A promise of future benefit has limited influence on current/future association, with the influence inversely proportionate to the length of time until the benefit will be given and directly proportionate to the degree to which the female trusts the male.