> Why is it limited to a “who”?
"Who" implies a human. The topic is human rights. I'm not sure I follow what this particular question is trying to get at.
> Why is it linked to sex or gender?
Pro answer: It isn't. We've known this since prehistoric times.
Con answer: Because only a man and a woman can create a new human life on their own.
> Is this a singular love or is polygamy a human right?
Age differences would have made for a better example here. But my answer would be that this question is that it frankly doesn't matter for my example. I.e., one does not need to answer the question of "how many people should we allow someone to marry" to answer the question of "should people be allowed to marry someone with the same genitals as themselves". Indeed, our global reality reflects that.
> What’s the fundamental human right?
What I previously stated: "loving who you want without regard to sex or gender". Notice that it does not say "without regard to singular vs plural love" or "without regard to reciprocation" or anything else.
> Shouldn’t we be worried that these so-called human rights are being newly discovered relatively recently?
That's... how it works.
We (or at least Western society) don't enshrine rights into laws and constitutions until we realize something we ought to be allowed to do freely is being unduly infringed upon. That's a process that doesn't stop.
We should be worried when we stop "discovering" new rights, not the other way around. Humanity has yet to produce a generation that "has it all figured out" with respect to fundamental rights, and there is no reason to assume we ever will.