Contrary to your assumptions, it is not the case that any law expressing some kind of moral disapproval is now prohibited. Virtually all of our laws, including those against murder and tax evasion, express moral disapproval.
This is how it works. If a law (such as those against bestiality) do not burden a protected group, then rational basis review applies. Bestiality practitioners have never been recognized as a vulnerable social group. That means that courts will uphold anti-bestiality laws as long as there is some rational basis for them. There is a rational basis for laws against bestiality, because animals cannot give consent and it can harm animals. No court would disagree with this.
Regarding polygamy, as I mentioned, bans on it do seem to express animus towards a politically unpopular, vulnerable group, which means that strict scrutiny review applies. This means that courts can only uphold if it is narrowly tailored to serve a compelling state interest. This is a much harder test, but ultimately I think courts will uphold it. This is not to say they should — it’s just a prediction.
Health, I agree that promoting homosexuality could reduce the pool of mates (depending on the relative rates of homosexuality among the genders). I also think the ban on gay marriage not only has a rational basis (many bases, really), but also is narrowly tailored to further a compelling state interest. However, as a practical matter, courts are not adopting these arguments, in part because, unfortunately, it’s not generally seen as politically correct to make arguments against a group (such as gays) seen as an underdog. That’s an unfortunate feature of our country’s messed-up political discourse.
I enjoyed reading Scalia’s dissent in the DOMA case, and I largely agree with him, but I don’t think this case will directly lead to the Supreme Court forcing all states to have toieva marriage, at least not anytime soon. Kennedy’s federalism concerns will keep that from happening, I think. The important thing is who replaces the next few justices who retire.