If a man and a woman love each other and want to live their lives together they get married. If you have two people of the same sex and for, whatever reason, want to live their lives together that is not marriage, call it what you want, a civil union or whatever but it does not fit the original definition of marriage. To call it marriage is like calling the color green, yellow because, that is what you want it to be, even though it is still yellow. This is not a comment on gay people, straight people or those in between.
The words from the Bible say, a man and a woman leave their family and cling to each other. The meaning of marriage from the Bible is much older than from the new versions of some dictionaries.
There are different views on the subjects of being gay, straight and those that were gay and turned straight. So it is not an easy subject and I don't claim to have all the answers. Waltz Disney may have some answers, they have recently been sued by the parents of a gay person who turned straight and wanted WD to provide sensitivity training for their employees on how to deal with people who were once gay and turned straight. It is a strange world out there so lets not change the meaning of words just because we want them to be different. Make up new words for new things, don't change the meaning of what already exist in order to fit into some notion you have of the way you want things to be.