That's... slightly confusing because they use Futa X (caps) and x (lower case) as the rather than Z to avoid confusion

But basically, if Z wants to be fertilized, there is the necessity for male Y ejaculate into the urethra of Futa Z at the same time Futa X ejaculate into the vagina of Futa Z.

Apart from having both organs of the sexes, futas also have an extra passage to allow semen to backflow from the urethra into the womb.

But because the males are really tiny, normal ejaculation will not be able to reach the egg, so during intercourse, it becomes a necessity for Futa Z to climax, so Male Y's sperm can ride on Futa Z's semen as it backflows into the passage to the womb, allowing Futa Z to get... fertilized

Why is Futa X even necessary? I mean it's technically self-impregnation at this point because it's the sperm of Futa Z and Male Y

I mean, it would make more sense if it was Futa X that was the target of insemination, as Futa Z has Male Y ejaculate into her urethra, then unloads into X as Z and Y's sperm cells do their thing