Traditional heterosexual marriages, for example of a widow after children are grown would fall into the first category. Heterosexual couples without the intent to raise children would fall into the first category until the wife became pregnant. Failing to qualify for the more rigorous standards for marriage would call for adopting the children when born. Child rearing is that serious of a commitment.
The entire purpose of marriage is to protect the development of future generations. We have allowed marriage to be watered down by no-fault divorce laws so that any ol' fling qualifies and if kids are born, oh well. We should focus on correcting that problem and let the disinterested, the infertile, the selfish, and the depraved live under the tenuous standards we have today.
Interesting post. At least you've thought about the issue.
Now, to your point: what happens when the husband turns around and says, "we agreed to a non-reproductive marriage, and now you've unilaterally increased the stakes -- and your own legal rights vis-a-vis mine, not coincidentally -- by getting pregnant without my assent. You practiced to deceive me. Fraud!"
Also, if you simply say, "well, the State will take the children away from non-qualified parents," then what you've done is destroy an ageless right of people to reproduce, and made it a revocable license extended by the State in return for certain policy objectives being kowtowed to first. Such as (arguendo and ad absurdum, as an exercise in destructive testing of the idea) the right of the State to insist that parents agree to raise their children as Greenies or Democrats, in return for the license to bear children.
What about that?
If we're going to strengthen marriage, we either need to tighten divorce laws, change the underlying culture or both. Endorsing any sort of homosexual unions because of the sad state of heterosexual marriage is moving from the stick house to the straw house.