The way I understand it (I may be wrong), but it's not technically legal, but some local jurisdictions and states claimed it treated gays as second-class citizens so are marrying couples anyway. Some people (including the president) are against this, and want to add an amendment to our Constitution (the document that "secures the blessings of liberty to ourselves and our posterity") banning it altogether.

As for who marries them, I don't think there have been religous church weddings (I may be wrong), but surely anyone can be married in a chapel or elsewhere by a minister (anyone can be a "minister" ) or a justice of the peace, etc.