I've asked people what "natural law" is before and never really got a satisfactory answer or one that didn't involve reading whole theological texts. Can you do any better?
And here I was thinking that "natural law" involved things such as the laws of physics...IE universal law.
As a result
The problem is same sex marriages want children and that violates natural law.
Is false. There is nothing we can observe from experiments or evidence that precludes same sex marriages from wanting children. Evidence I've seen suggests strongly that the opposite has a very high likelihood (for it not to be true, every gay couple claiming it would have to be lying, which is unlikely).
The act of giving birth directly without heavy surgery is constrained to women, but otherwise I similarly see no evidence as to how this supposedly violates natural law.
I see no need to flame your statement, just to point out that it isn't supported by evidence.
If, as an alternative, you are claiming the above violates "natural law" as defined by religion, you are claiming this action should be restricted to others regardless of evidence of its merit, for no reason other than "I don't like it" or "because I said so". In creating laws for society, such is not a constructive basis for restricting action.
The only thing that "violates natural law" is children not having access to loving, stable parents and caregivers. It's not like heteros are exactly excelling on this issue, particularly when they're caught trying to artificially limit access to stable legal relationships amongst parents and caregivers.
If you go by the "universal law as defined by nature", then even that does not violate natural law, neither do murder/heinous acts. However, heinous acts have negative utility while properly caring for children does not (it has very positive utility to humanity, and by extension to individuals), so it would be reasonable to make murder illegal and encourage raising children well.
If you go by the "deus vult" reasoning, then hell you could say any of them are good or bad, and the underlying rationale for doing so would be identical (some being we can't comprehend said it's good so it's good by definition, off we go then)...which is why that rationale is weak as a basis for decisions or law.
Edit:
As Triewd defines "natural law", same sex partners wanting children only violates natural law if none of those in said marriages actually want children, and they simply claim that they want them (either by lying or as an honest mistake). I feel the evidence against that is crushing.
But if we reject the notion that they're all mistaken/lying, then observational evidence suggests to us that they do want children and that this does not violate natural law.