What's wrong with marrying a toaster?
Nothing wrong with it at all that wouldn't be fixed if the government simply stopped discriminating against people on the basis of marital status. In our future IOT-enabled kitchens, when you get some eleven-year-old to upload an app into your toaster that hijacks its lame-ass-marketing "Your toast is done now, sir." loudspeaker and makes it say "I do.", that won't actually force any of the rest of us treat you and/or the toaster any differently. So why would it be a moral issue?
Why can't people sell their kidneys?
Same reason people can't contract to pay 1200% interest on a loan. Nobody would enter a deal like that except out of desperation. So if a market develops, participants are practically guaranteed to resort to illegal enforcement measures. Ounce of prevention/pound of cure.
...or their unborn embryos?
What's the problem? People already sell their sperm and their ovums and it isn't wrecking civilization. Why anyone would want to buy an already fertilized egg out of somebody's womb instead of just doing in vitro fertilization I don't know, but if that's what floats somebody's boat, why should the rest of us object?
They don't have the honesty to agree that one person's slippery slope is another person's libertarian trail blazing for the rights of people to marry whoever they want, to do what they want with their own body, to enjoy their short life with how ever much hedonism they like.
Well, apart from Loren there aren't any libertarians in the thread. Liberals trail blaze for people's rights too; progressives trail blaze for whichever social engineering they're in the mood for; social democrats trail blaze for their own right to vote out old social arrangements and vote in new ones; and so on and so forth and such like...