Gravity (phenomenon) is neither a theory nor law, in a similar way that a cat is different from a picture of a cat or a dictionary entry describing the word cat or our collective understand of what a cat is. At the same time, gravity is also BOTH a theory and a set of laws in the same sense that you could point to a picture of a cat and say “that’s a cat right there”, and no one would correct you. The distinction seems silly, but it is important. Theory, law, etc are structures/lenses through which we understand and predict things. A sort of formalized collective metacognition is the basis of science, and this is why we have these terms and distinctions. And theories and laws are fundementally different things in a way that’s may be best expanded by critically reading the resources provided by the other commenter.
It seems to me that all of the reasons they provides are all reasons to get married. Especially raising a child, given the privileges that are afforded to married parents in a lot of places (especially in the case of adoption, or IVF using a stranger’s genetic material). Something doesn’t have to require marriage for the benefits of it to outweigh the cons for a specific situation.
The question seems to me to be kind of confusing. What alternative are you comparing it to? Some sort of local structure like domestic partnership?