To me it's not a turn off for a gentleman to have a large stomach, but getting into a comfortable position is a bit tricky.
I'm rather small at just under 5' and 95lbs so the men with larger bellies are a little harder to maneuver with, but no matter the size or shape of the participant everyone just needs to find a comfortable position to be in.
It is definitely easier to move around and to get fun with clients that are more "in shape."
Creativity is your friend in the bedroom
and lots of pillows for propping helps too!
~Liz