given that a zygote will exhibit symptoms over a series of a few months that a sperm or egg will not exhibit independently in a million years, I'm not sure where the flaw lies ...
All right, anyway, one can't be "pro-egg"; haploids simply won't develop. It's a false case of regression to absurdity.
That's not known for sure, and anyway, it assumes that the "humanity" is defined as/gained with "autonomous" viability, or more precisely, the level of viability that the zygote has, tautologically, whichever it is.
The main problem is that not even the newborn baby is really autonomous; he or she depends totally on someone else to feed and nurture them, or else would die. Some weird hypothetical cultures could compare the act of voluntarily left the baby starve to death to deny the fecundation that the egg normally needs to survive and follow its natural life cycle (which could be stated either in a "anti-baby" or in a "pro-egg" way).
So it emphasizes the tautology, that is not merely a question of capacity of autonomous development, but developmental capacities particular and natural to the zygote stage.
Furthermore, the unfecundated egg can indeed start to develop itself into a fetus. It's a rare event, not known to ever have resulted naturally in a healthy mammal newborn, or in a natural mammal newborn at all (in all the confirmed instances I've read that it happened in humans, it had to be removed/aborted since it was not developing in the uterus, but near the ovaries, as often happens with normal zygotes as well, in which case they also have to be aborted), but I think that the possibility of rare natural occurrences can not be entirely ruled out. It was already achieved artificially with mice, and is viable (or even the exclusive form of reproduction) in other vertebrates. Every now and then we discover of something new, from new instances of parthenogenesis itself, to many weird things that were not thought to be possible (in the early 2000's some Italian woman got pregnant again a few months during an ongoing pregnancy).
If we hold that the development potential means something about "humanity", I think that this not well known potential of unfecundated eggs has some sort of implication. Fecundation or lack thereof can't be seen as a clear-cut divisor of "humanity".
From this perspective, one has to ponder whether eggs are in a certain way not much different from a zygote, that for some reason, would need a very important (and cheap) assistance to assure a higher chance of survival and healthy development, or whether zygotes/embryos are really all that "humanly" imortant just because they have about 40-60% of chance of developing completely.
If it is not "absurd" that a cell in which another cell entered and added some polymers should be considered an human being/person, it does not clear why is absurd the idea that the very same first cell could be human prior to the addition of (half of) the polymers; they're both living cells, they're
the same cell, differing in millionth of a gram of some internal chemicals. Of course, I hold the opposite opinion, that neither cell is an unicellular human being. If one is absurd, both are likely similarly absurd, unless there's some really good reason why these polymers have such an "humanly" importance.
(Which is not just to say that they're needed to the normal development; I belive that keratin also is, but no one makes a case that we're not humans until the synthesis of keratin in some cell)
