'That don't make it a good idea'(as Chris Rock might say), and that doesn't make it something our public would choose to recognize/honor as 'marriage'. And defining all or none of these behaviors as marriage is a 'public policy question' for citizens and legislatures, not a 'constitutional, civil rights question' for courts.
Re the 'public policy question' of calling any of these behaviors 'marriage' and according them public benefits, the reason we haven't is that most of us don't believe these behaviors do any good, much less serve the traditional purposes/function of natural marriage.
For a rough analogy, a group's demonstrated enthusiasm for setting fire to items of their own property isn't likely to earn them a public standing as licensed firemen.
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.