Chairm's comment #11 on this Family Scholars post expresses perfectly how banning SSP but allowing SSM will change marriage:
"Thusfar, SSM advocates have claimed that their reform would just tinker with the edges, not the core of marriage. It seems to me that if the right to procreate with one's spouse would be denied as a result of state recognition of SSM, then, marriage will indeed be replaced by something else."
If marriage doesn't mean that the spouses have a right to procreate together, and if some couples are denied the right to procreate, then all of our procreation rights are in jeopardy. In this age of genetic screening and increased reliance on IVF, that's a very grave situation.