The bit about "doing any good" I think harms your argument. According public benefits to homosexual relationships is not a bad idea just because those behaviors "do no good."
Here's my argument. It is solidly better for everyone if a greater percentage of existing homosexuals practice safe, non-promiscuous sex with each other. Formal vows to be said by homosexual couples may push things in that direction. One can agree with all that yet still staunchly oppose judicially imposed accordance of marriage benefits to gay couples.
Seriously, it’s not about “getting married” for the activists -
it’s about undefining marriage, removing the pillar of our society.
Evidence - only 5% of “gay” couples get married in Canada where it’s fully legal.