My first preference would be that same-sex couples should be allowed to marry like straight couples can. If that is not allowed, then I absolutely think that civil unions should be allowed.
The reason for this is because there is a need to allow same-sex couples to have legal rights similar to those of married people. For example, it is important that same-sex partners should be automatically allowed to visit one another in the hospital or to have the power to make decisions for one another when one of them is incapacitated. They should be able to be the beneficiaries of one another's pensions or to be on one another's health insurance plans. All of these things would make life easier for same-sex couples and would contribute to the stability of their households.
I do not think that gay marriage is going to harm America so I think they should be allowed to marry. If that is not allowed, then civil unions should be so as to give them many of the rights associated with marriage even if we will not allow them the ability to officially marry.