Who says that marriage is what it is? Is it the duty of government to maintain tradition? Is it the view of the government that marriage is what it is? of the writers of dictionaries? of the churches? Of a consensus among citizens? Who has decided what marriage is?
How is that decision in the purview of the federal government?
Things change with time. Words change meaning, mores, values, goals, ideals, all change with time. It is not up to the government to decide what may and may not change.
If the law doesn't say that abortion is illegal, how is outlawing it enforcing the law?
As for instances when abortion is OK, even according to most "conservatives", there is rape, incest, or life threatening medical complications. Are we to allow the government, or the individual to decide when such instances are justified?
The conservative philosophy would leave that difficult decision up to the individual, wouldn't it? I can see a controlling, powerful government deciding who must continue a pregnancy, but certainly not a limited, conservative one.
