While on the subject of Jenova (and Sephy), I also like to share this idea I have which explains why I think Jenova is a 'she':
Shortly after its arrival, and while infecting the Cetras with its cells, it had observed particularly that women have the capability to carry and nurture their young, as opposed to men, who have no direct interaction with their offspring while in the mother's womb. Thus, it had thought of a way to duplicate itself, to reproduce while at the same time, blending among the populace for protection-- and that is to turn itself into a woman, at least superficially. Couple that with DnD's idea of merging Spirit energies within a hybrid fetus: Jenova will now be able to give birth to a new breed of 'Sephiroths'-- more powerful than itself and more capable of bringing destruction. (This idea is also inspired by an Anne Rice novel I read, 'Lasher', in which the male entity tries to impregnate women, but due to them being a 'weaker species', they died with the offspring as a result).
After being defeated by the Cetras, it used its remaining energy to transform itself, but Gast found it before its transformation is complete (hence the appearance). Ironically, Shinra completed its purpose through Project S, although via a different method. (Also, I'd like to think that should Jenova had succeeded and was able to carry the infant on her own, it will be more powerful than Sephiroth, who was born to a human mother).
Am I making any sense? Lol sorry