Marriage is a sacred institution, it is meant for between a woman and man. Lobbying for same sex marriage is more of a mental diversification and should not be associated with the sacred word 'marriage'.
God has created man and woman differently associated with unique feelings for union. Marriage causes this union in a disciplined manner. It is necessary for maintaining growth of humankind on the Earth. It is the nature's way of doing things. Same sex union is something unnatural. Probably this tendency is noted only among human, not among any other living creatures those are maintaining its generation by union of male and female species for ages.