Learn English – n explanation for why both “pop” and “pops” are used to mean father (singular)

From the Oxford dictionary online, via Wikipedia:

The combining form uni- does normally mean 'one, having or consisting of one': it comes from Latin unus 'one'. It forms words such as unicycle, a term for a cycle with just one wheel, and unicellular, meaning 'consisting of a single cell'. And in fact the 20-volume historical Oxford English Dictionary contains entries for the words unisexual, meaning 'of one sex or relating to one sex' and unisexuality, meaning 'the state of being unisexual'. Both these words date back to the early 19th century.

Unisex is a much newer word: it was coined in the 1960s and originally used in relatively informal contexts. Its formation seems to have been influenced by words such as union, united, and universal, from which it took the sense of something that was shared. So unisex can be understood as referring to one thing (such as a clothing style or hairstyle) that is shared by both sexes.

Yes, that’s right. In general, it turns out that sometimes either is distributive, essentially meaning both or all, and sometimes it is exclusionary and so applies to just one out of the set.

Your question is whether it would be understood to mean just one or if it would mean two in the sentence:

Either of these options will do.

The answer is that here it means that just one option suffices. It means “any single option”. If you have any doubts about how it will be received, you can always write any one option or some such.

For the most part, I think the “each one” or “both” sense applies to natural pairs. Here are some OED citations for the “both” kind of either:

• 1762 Falconer Shipwr. Proem 40 ― The fierce extremes of either zone.
• 1820 Scott Ivanhoe iii, ― There was a huge fireplace at either end of the hall.
• 1842 Tennyson E. Morris 37 ― Either twilight and the day between.

Note that either meaning “both” is the oldest of the various senses the word has come to mean historically. It is somewhat uncommon these days, but by no means wholly obsolete.