The Omskivar
chah, dude
Why should it match their gender? It's really common for real voice actors to have a different gender than their character! You know, like, Ash was voiced for years by Victoria Taylor.
Also, there are lots of trans* people on the forums! Including people who are non-binary genders (who might want to try for binary characters!). So that's kind of a sketchy territory. And what if there are way more $gender characters in a chapter than people of that gender who want to participate that round? Or what if it's a Mew chapter, but no agender people are voice-acting? It would have to be put in hold indefinitely. ("all legendaries who are genderless in the games [...] don't have a gender as we know it.")
Giving people an incentive to try for lots of roles is good, but deciding it by gender is silly. I think everyone can talk about things like "hm, I was hoping we could get a deeper-sounding voice for Mark. Anyone up for it?" without making it about gender! Then if someone happens to be able to do a good deep voice even though they don't have the same gender as Mark, they can go for it (I don't think Mark has a deep voice, but). There's no need for people to be made to feel voice-shy because of gender! It would be a lot nicer if it were comfortable for anyone.
Amend the statement however it is seen fit, that's fine, in any case the idea was for each potential voice actor to audition for as many roles as possible, to avoid neglect of minor(YMMV) characters and/or missed potential on unexpected roles. In any case auditions seem the way to go.
I feel like the idea of people getting bored with the roles and dropping them would mostly be not a problem except for the main cast, namely Mark and May; Mark especially could be quite jarring if suddenly his voice is noticeably different.
There's also the issue of a narrator, who would be even more jarring to change midway, but would also be potentially the least interesting part to play, considering it's mostly a straight read.