well it doesnt have to have EVERY trope, but when you are looking at a fairly large number of species and trying to come up with an explanation for for each of their reproductions, and dont want to repeat, then you are bound to use asexual (as well as 3 gender)
Why not repeat? Evolution tends to figure out what works and runs with it. And what works for one sentient animal-like life-form is probably at least going to be close to what works for another. Fur obviously evolved at least twice in the Animorphs universe, as did scales and feathers and lungs and hearts and stomachs and internal skeletons and eyes and ears and hands. Heck, every single Animorphs species uses aerobic respiration!* Why is it any harder to believe that a two-gendered mode of reproduction is just as adaptive?
Given that asexual reproduction exists on earth, yet has never taken hold with any of earth's most dominant species, it's quite plausible to assume that it would be equally rare among extraterrestrial life-forms, as well, especially advanced ones. Granted, it's probably not completely unheard-of in the entire universe. But it also would not be nearly common enough that we could simply assume that, out of any random collection of species, one or more must reproduce asexually.
*Yes, including Yeerks. According to HBC hey start to suffocate when buried in mud, and according to Cassie in #29 they breathe through their skin.
been thinking more about the andalite one now, due to ax not understanding why humans cover up, it would seem that whatever sex organs andalites have, they are internal and non fetishized.
I'd agree with that. Especially given the fact that none of the human Animorphs have never pointed and laughed at Ax running around naked all the time.