To be honest, I think you could make a pretty good case (not an overwhelmingly complete case, of course) that not all of the currently gendered pokemon should really have genders. Two examples that come to my mind are Shroomish/Breloom and Foongus/Amoonguss. Both are based on fungi, and many fungi reproduce by asexual means. In other words, they don't need to fertilize each other, they just make new clones of themselves. There doesn't seem to be any need for these two mushroom-creatures to have both males and females.
But really, you could make the same argument for pretty well any plant-based pokemon. There are a few plants out there that have definite male and female individuals, but the majority of them seem to have both male and female parts on the same plant, making them both male and female. Besides that, pretty much any plant that can put down roots has the potential to reproduce by having new individuals splitting off from the "parent" plant. Either way, again, there's no reason for pokemon based on these plants to be either male or female. To be most accurate, they should either be both male and female, or be genderless but able to breed with other genderless plants in addition to ditto.
So, no. I don't think that all pokemon out there really need genders. Being alive, able to breed, and able to reproduce doesn't automatically indicate that you have a specific gender.