Where to begin? In no particular order…
D. D is a conservative republican heterosexual woman. She’s been married twice; divorced, then widowed. She’s on the market. Actively. Has never wanted children (in part, this led to the end of marriage #1), but very much likes being married.
M & K. The brother of a friend of mine, and his (male) partner. Not married (their state does not allow it), just together for 27 years so far. They adopted 2 kids—AIDS babies nobody else wanted—and loved them their entire lives, and mourned their inevitable deaths. Among the best people I know.
V & P. I love these people. A heterosexual married couple, they never had their own biological kids—I don’t know if this was by choice or not, and it’s none of my business. They adopted 3 kids from halfway across the world, and received anonymous threats because their children had darker skin than they did. Their kids grew up to be wonderful adults; I just ran into their youngest a little bit ago (she was in my daycare when first in the US) and I was delighted to see what a wonderful adult she had become.
V & H. Different V, of course. These were my neighbors, quite possibly the best neighbors in the history of neighbors. Two women, one son. He’s now engaged (to a woman, not that it matters), and a wonderful man; for a time, he was my son’s best friend (despite being a year older and in a different class in school). V & H (I can say this, since I am not really identifying them) each wanted to marry the other, but had to move to another state (for job reasons) before either of them popped the question. As of today, they are not married, but they have raised a fine son who is about to be.
S & V. Another different V—it seems I know a lot of V’s. Two adopted kids (from Korea), one biological. A wonderful family.
V & H. completely different from the V & H above. V was divorced, with 3 kids; H proposed to him while she was pregnant with his 4th.
B & J. my uncle and aunt. He was a pastor. Three kids (my cousins), then he left her for his true love, a catholic priest.
H & M. Cuttlespouse’s uncle and aunt. Again, a pastor. Again, three kids. Stayed with the marriage; abused the kids. They don’t speak to him any more.
C & C. And K & Y. And D & D. and C & L. Heterosexuals, married, with kids. Three out of four of them are in favor of same-sex marriage. The other couple have religious reasons to oppose.
I could go on. Easily.
Relationships are more than a spectrum; they are a spectrum of spectra, along multiple orthogonal variables. Variability is the only constant; there are exceptions to every rule. Heterosexuals, and homosexuals, adopt. And have surrogates, or IVF. Or choose not to have kids at all. If you want to protect children through the institution of marriage, don’t look at who is getting married; look at what people do. There are bad parents of every stripe; there are good parents of every stripe.
In my opinion, there are two paths here. You can start with the constitution, and try to justify unequal treatment under the law… and fail. Or you can start with the evidence, trying to show systematically better parenting among heterosexuals than homosexuals… and fail.
Ok, there is a third path. You can recognize reality. But, really, who expects the courts to take that path?