It's a cliche'd mess. Riddled with kind of ridicilous dialoge, The premise starts off fine enough mewtwo needs to mate and chooses ash for his compassion from the first movie and mewtwo strikes back, then it just becomes a cliche'd mess bringing up that ash is a boy and that mewtwo's power can change that, the other pokemon only distract from the whole thing even if you are into the fetishes on display. It's clearly just picking some of ash's favorites over what he might legitmently have at that point in time, while also just using them to overly reinforce the thing that doesn't need to be said redundantly.
I'm not invested in this, cut the redundant dialog back, mix it up a little so it doesn't seem super cliche, and be a little more 'into it' as it feels kind of lifeless and honestly sexless for a sexual fanfic.