Yeah, is there any specific reason they're against it? If it's because they're claiming it's childish, then I'm gonna be blunt and say that that's pretty dumb. Despite the age rating, and the ages that the franchise is marketed towards, there are several elements of the game that are implemented with the older fans in mind, and Pokemon is really a game for all ages. Even though the anime may be really childish at many points, it doesn't mean the whole franchise is. They shouldn't really dictate what games you want to play either, if it's on simple matters like that, and if they claiming that Pokemon is childish really is the reason, then bring up just how many people older than the marketing age that are playing it.
If it's about the whole morality, and with having Pokemon fight against each other, then I don't really think they should make such claims without getting into the whole thing first. PETA already brought this up several times, and haven't recieved much praise on their claims, and if that's the case, then there was a hefty discussion somewhere here in Pokemon General about how this wasn't really a point that should be brought up about Pokemon.
If it's anything else, then please elaborate so we have a better idea of what's going on :/