Yes, it's true, mushrooms are grown in sterilized horse manure, but the key word there is sterilized, which means that the medium is even less worrisome than normal soil. We've recently seen many problems with produce grown in soil, but not with mushrooms. As an undergrad in Horticulture at Oregon State, we visited a mushroom plant in Salem and even tho there were big, steaming piles waiting to go into the mushroom beds, there was NO ODOR at all. And, inside in the darkness, there was just a warm earthy odor.
Also, mushroom are little sponges: no matter how quickly you try to wash them with water, they WILL absorb some and just add to your problem. I always saute mushroom slices for a pizza in a little bit of butter until they are nice and dark brown... not quite the duxelles that art mentions (which is another good idea) but getting there. If you use very little fat, they will brown and seal in the moisture rather than release it. It usually takes ~20 min over med-low heat to attain a nice dark brown, stirring once in a while so that both sides of the slices are well browned. I also don't buy white button mushrooms, I buy cremini because, since they are baby portabellas, they are drier and have much better flavor.