Note, ectopic pregnancies and fallopian pregnancies are not in the womb, but outside the boundaries, anatomically speaking. Only when the continuation of the pregnancy will result in the death of both the mother and the baby is the termination of pregnancy justifiable.
As for: Therefore, to the extent a state understands and values its own continuity, the state will naturally be concerned with the most natural context for sustainable procreation: Marriage
These matters were handled by the churches at one time. If you dig far enough back into geneology, the records of birth, death, marriages and such fall primarily into church records, not government ones.
Legal matters of property were recorded by government, primarily for the purpose of taxation. Heirs and assigns were recorded for the purpose of recovering such debts as may have been owed the state, and for the purpose of levying taxes upon them subsequent to the demise of the former owner.
While the church may have been concerned with the welfare of the children, such has not been the concern of the State until the last hundred years or so. Legal obligations were, again concerned with taxation and succession, moral obligations were the focus of the Parish.
Marriage regulationa have focused on heterosexual marriage because that has heretofore been the only variety recognized by either church or state, as it should be. It is only in the past 40 years or so that perversion has supplanted the presence of open reference to God in public and even private institutions to the degree that homosexuality has become an 'open' issue at all.
I would note that the attempts by the state to take over the responsibilities of the church have only led to a variety of 'morals' which can only survive as long as they are supported by popular vote. The Church has scripture to guide it, and without the convolutions of twisting the teachings of the Bible cannot condone anything but heterosexual relationships.
Well, the history of state regulation of marriage and its points of contact with legal systems actually goes back as far as Hammurabi. The problem using the argument that it is strictly a province of the church is that the church, in various periods and locations, was for all intents and purposes coextensive with the government. Our modern quasi-bifurcation of church and state is a relatively recent phenomena and took a long time to evolve to its current condition, and I would contend that part of what is happening is the modern state is attempting, with some difficulty, to learn how to function in an area where the absence of religious governance is making the procreative interests of the state more difficult to secure. Therefore, I would still contend that, as long as natural law is the basis, government may still reasonably and beneficially involve itself with questions of marriage.
Consider the consequences if it does not. There is a dynamic of mutual exclusivity in the marital paradigm shift from primarily procreative and thus exclusively heterosexual, to the postmodern notion of arbitrary and primarily pleasure driven relationships. In the former, there are natural human propensities that foster a self-sustaining system that is beneficial to the state. The latter, as demonstrated by the Netherlands, tends over time to a disintegration of the whole concept of marriage. Therefore, if the state does not take sides, the private parties will end up in a winner takes all struggle, with no referee. Heterosexual practices are already actively being deconstructed by, among other things, carefully calculated lawsuits, of which the net effect is not the increase of equal rights but the diminishing of the protected status of the procreative paradigm, which in turn results in higher risk of harm to children.
This is a nontrivial effect and should be a consideration in what position the state takes. A state position of theoretical neutrality amounts to a decision of the state to do harm by doing nothing. Therefore, neutrality, as posited by some libertarians, is not an option.