yytz,
The Court in Romer, Lawrence and Windsor never described itself as using strict scrutiny or even intermediate scrutiny. They are all rational basis cases. Windsor didn’t explain that it was applying heightened scrutiny, because it wasn’t. I would guess that Kennedy was leaving himself an opening (by not declaring it a fundamental right or homosexuals a protected group) to leave the decision up to the States in a future case (hence the federalism dicta).
My broad description of these cases actually comes from an interview with Justice Kagan who explained them as being based on this idea that moral disapproval or animus cannot be the primary basis of legislation.
Left unsaid, but I think obvious, is that the legislation would only be problematic if it infringed on somebody’s rights. But those rights need not be “fundamental” (in the classical sense) and, therefore, almost all prohibitions qualify.
“Unpopular group” has not been a legal designation under the Court’s precedents. Anyhow, it is hard to imagine a more unpopular group that practioners of bestiality.
Agav, Windsor is really a 5th Amendment Due Process case, applying the “equal protection aspects” of that clause.