One year ago, Kieth Olbermann said it just right:

What I don't understand is how two consenting adults loving each other threatens anyone else? My marriage isn't threatened if gays can marry!

Don't have much to add to this except I am also a straight man from a family of straight folks who support Marriage Equality and just don't get this whole homophobic anti-gay marriage movement:

What I don't understand is how two consenting adults loving each other threatens anyone else? My marriage isn't threatened if gays can marry!