I'm not familiar with this latter scenario. A man being sterile is nothing like this algae's situation, as it has nothing to do with "pairing," he's simply unable to produce children with anyone.

I don't know, in my experience people are sure picky about procreating with random strangers (and the ethical concerns regarding the output of the study are... significant).

From people I've known undergoing fertility treatments (and from my own limited research), the issue is frequently traceable to a specific cause in one or both partners (although it appears to be unexplained in about 20% of couples who seek treatment, which would be a small subset of the general population).