Normally the slippery-slope style argument predicts (sometimes but not always fallaciously) a kind of political or moral disaster if a certain kind of thing is allowed. For this reason I sometimes wonder whether such an argument should be called "argument from permissiveness." For, if we permit gay marriage, then all manner of things must also be allowed (triple marriage, quadruple marriage, limited liability companies, etc.). They serve usually as a warning against something relatively minor and incremental: if they get their foot in the door, then you will have to contend with consequences x, y, and dreaded z!
On this topic, the blogosphere is a aflame with Orrin Hatch's dire warning about the consequences of socialized medicine:
HATCH: That’s their goal. Move people into government that way. Do it in increments. They’ve actually said it. They’ve said it out loud.
Q: This is a step-by-step approach —
HATCH: A step-by-step approach to socialized medicine. And if they get there, of course, you’re going to have a very rough time having a two-party system in this country, because almost everybody’s going to say, “All we ever were, all we ever are, all we ever hope to be depends on the Democratic Party.”
Q: They’ll have reduced the American people to dependency on the federal government.
HATCH: Yeah, you got that right. That’s their goal. That’s what keeps Democrats in power.
There is also a little bit of "you're only saying that because. . . " in here: the Democrats only want health care reform because it keeps them in power. I think there are more pressing reasons to want it, such as the fact that our current system is killing us, but maybe I'm naive.
The weird thing about this particular slippery slope is that the consequence Hatch warns against is that people are going to like the Democratic party. Such will be their adoration that they abolish by their votes the two-party system.
In the first place I think that's very unlikely, but if it were likely–and if Hatch weren't just lying–he'd see that he has just admitted that people would embrace the idea of "socialized medicine"–if they didn't like it,they wouldn't continue in Hatch's fantasy scenario to vote for Democrats.