Currently, women are federally allowed to change their surname to their husband's on a marriage certificate. Most states to not allow the husband to change theirs to their wife's, and such a restriction would complicate the legalization of gay marriage.
If two men get (legally) married in such a state, presumably neither could change their surname to the other's on the certificate. In the situations above, the husband would need a name change certificate as well, which varies by state.
If national law allowed the couple to choose which (if either) person will change their surname on the certificate, such complications are lifted and another LGBT and gender inequality.
Gay marriage is more or less coming, Mr. President. Could you please get this obstacle out of the way for these couples?