I'm not sure why your pumpkins have yellow bottoms. Maybe the plants are aborting the fruit? This could be because of stress, too much water might be a problem as well. I have dealt with that before.

Try setting them on a small board or upside down tuna can. This will keep them out of contact with damp earth. The yellow bottoms and rotting may not be directly related. Most of my pumpkins have a light spot on the underside. I don't try to change this. But if they have problems with spoiling, due to damp conditions or insect damage (earth dwelling insects), I put a little board under them.

George
Tahelquah, OK

What George said. I put a small piece of 2 X 4 under each pumpkin and winter squash that I get.