Magdalene, again it is a question of one group receiving a right that another does not. Currently, heterosexuals are allowed to marry. Therefore, lesbians and gays should be allowed to marry.
Currently, no one is allowed to marry more than one person. If this is ever made legal for one group, it should be made legal for all groups.
You are correct, however, that polygamy is a religious issue for many people. A church should be free to perform such a ceremony if it is congruent with the church's beliefs. The government, however, chooses not to grant legal status to polygamous relationships.
"I am open, and I am willing,
For to be hopeless would seem so strange.
It dishonors those who go before us,
So lift me up to the light of change."
Holly Near