Does or does not marriage convey certain advantages to people who are married?
It conveys costs. I find it humorous that gays are in such a hurry to sign up for the marriage penalty. I find it sad that the strongest case for SSM seems to be that no one can quantify what marriage advantages might be. I mean if that could be done, the argument for SSM vs civil unions disappears. So the strength of the argument is based on any advantage being nebulous. I see lots of people talking about lots of things, but no one willing to address that.
To me, marriage is the Blessing of God between 2 people, in the sight of "God and everybody." Religion not needed, as Adam and Eve had no religion when they were married, as the story is written. Most same sex couples aren't seeking that at all it seems to me, and those that do can find Churches to accommodate them. I don't think any of that falls within Gov't jurisdiction.
Civil rights are quantifiable and in the case of marriage are mainly economic, which is clearly within the jurisdiction of the Courts. Crying "civil rights" w/o being able to specify what those are is equal to crying "teh gayz." Gov't is pretty clear about what is within its jurisdiction, and SSM proponents should be able to specify what advantages they seek. Social acceptance is not within the jurisdiction of the Courts, and that is the only reason for pursuing marriage rather than civil union. (Assuming what civil rights are sought are specified and provided for under said civil union, of course)
Are couples who adopt or have children that are not biologically from both parents more likely to stay together?
What you're missing here is that staying together or not is dependent upon the integrity of the relationship. Couples that split do so mostly for 3 reasons: kids, money, and sex. Fundamental problems in all 3 areas in the same couple makes a very hard situation, with low likelihood of success. So the very same set of circumstances that will draw one couple closer together will break another couple apart. I seriously doubt this is substantially different for homosexuals, or due to factors like you are citing.
Either way, the bond a heterosexual couple experiences from the process of conception and childbirth is not equal in these "alternative lifestyles" you're bringing up. That is still something distinct, and part of what people allude to with the glib "sanctity of marriage" spiel. Neither does the fact that some heterosexual couples physically cannot reproduce make SSM in any way equal. The differences will still be there, and it's silly to pretend otherwise.
The catch phrase 'equal yet different' really doesn't pertain here. How about 'different and ok' instead? Wouldn't that ultimately be a lot healthier? Its certainly more realistic.
Another angle nobody has been willing to talk about is violence. When inter-racial marriage became the law of the land, someone else cited stats that 80% of US population was against it, and a decade later 94% were opposed. I seriously doubt those numbers, but its probably true that it increased the opposition, at least in some areas. How many people do you personally know who were victims of said violence? I know many, and I was living in a very culturally diverse and tolerant area when I knew them. Can we as a society really afford to jack around with this? Polls showing a slight majority favoring SSM still means some 150 MILLION people opposed, and a lot of geographical area where that sort of opposition tends to turn ugly. Comparisons to other Countries are irrelevant here. Each statistic is an actual person! Inciting further polarization is not wise.