Well, if we are relating the real world ghosts to Pokemon ghosts... Its believed real world ghosts can manifest and turn solid, this could also be the case. Also, in the Anime, Haunter is able to lick Charmander in the opening theme, something that could only happen if the tongue was solid, not a gas, magnetic field or such.
As for being poisonous to touch, real world humans and animals are known to build up resistances to poisons. Pokemon however, no evidence of this is shown, except in maybe the Manga with Koga using a poisonous gas against Red or something, however, many diseases and poisons need an open wound to transfer (As believed by many Zombie enthusiasts, which a person needs to have the virus enter the blood stream though an opening). Pokemon may also have the ability to control the secretion of poison, to ensure their trainer or friends dont become sick.
Finally... This is a childrens game, in a world where the only crime to speak of is Team Rocket, Team Neo Rocket, Team Magma/Aqua, Team Galactic, Team Plasma, Team Neo Plasma or Team Flare, where a Snorelax, the size and weight of a small car, can fit easily into your pocket and you dont need to keep your pants up with a belt afterwards, thanks to a red/white/black ball, and where there are no animals to eat yet Brock somehow grills something up that isnt a Pokemon. Trying to make sense of the reasons behind something, like I just tried to do above, just dampens the gameplay.