That's the question that I am asking you - why would someone who is only interested in the physical WANT to get married?
Most heterosexual men squirm at the idea of marriage. Men aren't the ones swooning about getting married - women are. Men aren't the ones wanting to settle down and have kids - women are. Most men don't want to get married until they find one particular person, and even then, some don't (George Clooney, for example.)
You, however, are claiming that these gay relationships are doomed to fail because they are only physical. Gay or Straight, if it is only about the sex, the person isn't going to want something permanent. They are going to want to be a "Sam Malone", sleeping with as many women as possible, not committing to one.
Yet, there are gay couples that want to be married.
So, how is that l-u-s-t?
Does lust want to make you get legally married now?