NO! All it takes is Nine Perverts in Black Robes to decide that that law needs to be changed, and they are perilously close to doing it. In fact I would not bet against it happening, and happening very soon, within the next year.
Yes. If they allow gay marriage, there is no logical reason why polygamy could remain illegal. Or incest for that matter.
After all, having hundreds of sexual partners is at least as destructive as having sex with a relative. How can they allow sodomy but ban incest?