For many men and women, getting married is the biggest thing that will ever happen in their lives. Certainly it has been in mine. I spent a lot of time trying to get into bed with women, and when I finally achieved it, it was a huge deal.
In a crude way, mating means you can go to the people in high school and say “see, I’m not gay”. Look at all those pictures on facebook of people advertising to their past their marriages and families. Making a kid is a sign that you may not be achieving much at work, but you can at least make decent offspring.
Gay marriage takes away that special state for many people who don’t anything else that compares in importance. I don’t believe I’m better than a gay man, but I think I understand that people are threatened in this way by gay marriage. Gay marriage takes away a distinction that makes them feel good.