Leftism is a curious phenomenon in that it simultaneously rejects every tradition, authority, institution, or convention which has the capacity to infringe on what it would call individual rights, yet has no clear metaphysical basis for asserting these rights.
Where do rights come from? Most far-rightists would agree that our rights derive from our duties and the duties of others (so that, for instance, if I take out a loan, I have a duty to pay it back and the right to do so, and the lender has the right to be paid back and the duty to accept my payments -- one man's duties being another man's rights). These duties, in turn, arise from human nature, so that men, whose physiology contains an innate calling to fatherhood, has a duty to protect the weak, provide for his wife and children, and so on. But leftists, being nominalists, reject the idea that there is any such thing as "human nature" from which rights can be said to arise; for them, the universe is a mess of radically particularized, constantly colliding entities moving randomly and chaotically through open space.
Most center-right types believe our rights derive from God -- that they are either granted by Him (explicitly or implicitly) or else derive from the duties He has proscribed for us -- but leftists, generally being atheists, don't believe in God, either.
Some say rights derive from the "innate dignity" of the human person. There doesn't seem to be any reason for believing in such a thing, though, if one rejects the idea of "humanity" as anything more than a useful heuristic for a completely dissimilar mass of beings about whom nothing can be meaningfully said, or if one rejects the idea of a "God" from whom man derives his dignity by virtue of his likeness to Him. So "dignity" is, at best, a useful supplement to a theistic or essentialist conception of rights; it cannot be a replacement for it.
It's in the works of Hume, Hobbes, and Rawls that we find the liberal basis for the idea of "rights," which is merely convention. Masses of moving entities, threatened with the prospect of collision, agree upon some framework of rights wherein they are permitted to maintain their momentum and speed but required to alter their trajectories -- harmonious movement being, in their minds, impossible to achieve and undesirable, anyway.
But conventions are negotiable, alterable, dynamic. Indeed, they must be those things; if they were fixed, they would not be able to serve the purposes which conventions serve, as adaptations to the social reality. Therefore "rights" which derive from them are not rights at all but merely concessions, the terms of a contract which may be renegotiated unilaterally at some future date without your consent, perhaps without even your notice.
Hence every decade or so we discover a new "right" which no previous society acknowledged, such as the right to manufacture and sell meth or to "marry" bits of fence post. And every few decades we cease to recognize a right which previous societies not only cherished but held to be the central and self-evident facts on which society was predicated, such as the right to speak religious truths.
And hence, when a leftist declares that something is a "right" -- or an "inalienable right" -- or a "natural right" -- or, most odious of all, a "human right" -- he is almost always confused and incoherent (at best) or deliberately lying (at worst).
He is not saying: "Man, by virtue of his nature, his dignity, and his status as a subject of God, enjoys, forever and without condition, the right to do or say or believe X."
He is saying: "I happen to like X, so I think society ought to tolerate, then endorse, and, ultimately, subsidize its practice."
Extrapolate this trend forward, supposing no interruptions, and it will end in something like a society where people are free to murder their elderly parents in their sleep or masturbate in a public park -- while those object to such outrages on religious grounds are summarily imprisoned.