By putting the decision to legalise same-sex marriage up to the states, those in more conservative states who wish to marry are not able to do so unless they travel to a state where their marriage is possible, and even then there is no legal obligation for their home state to recognise the marriage. Whilst homophobic people may be in favour of this approach, it is not fair to those who would benefit.
This is extremely unfair, and the United States is one of the few countries whose citizens have to deal with such a trouble. In Canada, for example, any two people from any province can marry, and their marriage will be recognised throughout the country. That is a far more egalitarian approach which I believe the United States should follow and would be greatly lauded for finally doing so.