If I had a child that was a drug addict - I wouldn't want drugs legalized just so she could feel better about herself. That's unfair to the rest of society.
I have many friends and acquaintances who are gay, and I respect them all as people. By and large, the gays I know contribute to society and are not interested in a political agenda--they are not "in your face" queers.
But what you propose, gay "marriage" is impossible.
The fact is that sexual activity is ordained toward childbearing and raising children in a familial environment. That's why the genders were engineered the way they are.
You've attempted to "make points" by pointing out this society's failures and anomalies, and then saying "well, if Bobby and Sue can fornicate/get divorced/prevent conception, then gays can marry..." Wrong. A false premise argument--producing a false conclusion.
It is irrelevant that the Enron management was composed of a bunch of crooks when considering accounting entries at, say, ExxonMobil. Right action, founded on right understanding of moral law, is what IS relevant.
Your relative's situation is difficult for you--and likely even more so for him. But that doesn't mean that his difficulty should militate erasure of the natural law, nor accord him the benefits rightly-ordered societies have conferred on married couples.