There’s been a lot of brouhaha since the Massachusetts Court decision around gay marriage. To me, the answer is simple and it sure doesn’t require a constitutional amendment.
Let the state’s only responsibility be to recognize the rights of couples to form civil unions. This provides committed partners the right to insurance benfits and household tax filings. It grants the right to access and make decisions about a hospitalized loved one. It grants the right to full legal status as a parent. The state should only recognize and grant these rights in the form of civil unions and should not have anything to do with marriages. The state evaluates couples of any orientation purely on the basis of whether they are fit for the legal obligations.
Then let the churches and other religious institutions perform the rite of marriage according to their customs and beliefs. Let them decide what “preserves sanctity. Religious groups then have complete power over the portion that is relevant to them - the spiritual portion. Couples that wish to have their household observed in the eyes of religion must find a spiritual home in which to hold their rite.
Heterosexual couples enjoy this freedom rather well as they can currently choose an officiant that best fits their beliefs. That may be a priest or a judge - the priest performs a rite while the judge simply grants rights.
To me, any other legislative solution confuses the roles and responsibilities inherent in the church and state separation. It is simply a question of right versus rite.