The main argument religious people have against gay marriage is that the institution of marriage is not being respected and degraded as it was intended to be a man and a woman in love. Now, whilst I personally don't agree with this and feel that any two people in love are equal, you have to realise that in its present state, marriage has already been tarnished and degraded by heterosexual couples (Kim Kardash+High divorce rate etc). I feel that the original entity that was marriage is actually more respected by gay couples than hetero. Gay couples seem to, in my opinion, genuinely want to be married for love. The majority of straight couples are the same, they want to be in love for the next month and a half and thus get married....
My $0.02