Marriage in my opinion is when two people have decided to sanction their relationship legally as a couple.
I would not get too caught up with laws and sexuality. Laws tend to change overtime. During the slave era in the U.S. black marriages weren't recognized. For many decades interracial marriages weren't recognized or "legal" and these marriages were between men and women! Each generation becomes more accepting of things that were "controversal" during the previous generation. Pre-marital sex, living together were once hot button issues. Viagra, KY- Jelly, and condom TV commercials would have been unthinkable during 1960s and 1970s! Today cigarette commercials are not allowed.
I think any paper your write has to examine our history of change and acceptance over the various decades. Illustrate the fact that more high profile people are "coming out" such as Anderson Cooper did recently, The government ending the military position of "Don't ask, don't tell", some states are sanctioning marriage between same sex couples, and the "Leader of the free world", President Obama stated he supports same sex marriage. In 25-35 years from now what is "controversal" today will be old hat like the government adding flouride to water supplies. There wil always bel something NEW to debate about.