Marriage contracts should be privatized. How assets are split in the event of a divorce, penalties for cheating, visitation rights...etc. All of this should drawn up in a private contract. Contracts can be individualized to the circumstances of each couple.
Certain contracts will probably become more mainstream and popular, and solidified as a brand. Such as Coke or Pepsi, a mainline standard contract everybody recognizes by name.
Some possible examples:
"The Alpha Male"
The man is expected to do all the work, and financially support the women and children. In return, the woman must treat him with respect, do the housework, spend lots of time with the kids, etc... The man entirely controls financial decisions. This union reinforces traditional family roles where the man, although dominant, has great burdens placed upon him.
"The Christian Standard"
The contract is recognized as a religious union, and is mediated by the church as a third party. The church holds a special court to decide who gets custody and how much child support is owed. Infidelity is punished most harshly and results in an immediate loss of custody.
"The White Knight"
The woman is entirely dominant in the marriage. She can leave whenever she wants, and take the kids and the house with her if he doesn't measure up to her satisfaction. In a divorce scenario, he will be entirely cast aside, forced to pay a large portion of his income with meager visitation rights if any. Meanwhile while the woman finds a new husband to act as a father figure for her children.
In my opinion, the state should still enforce a ban on bizzare unions. Such as marriages with 5 people, 7 people and a dolphin, 10 people and 2 horses, with any combination of male/female, gay, straight, bisexual...etc. These shouldn't be banned in general, only if they expect to raise kids in that scenario. It is my personal belief based on human evolution and biology that gay unions are extremely harmful to the psychology of children, almost as bad as single motherhood or single fatherhood. And I think they should be prevented from raising children through artificial insemination (adoption is fine).