My range on the scale has varied much over the years. I've been very svelt and I've been pretty damn heavy. Sex while pregnant was the only time it's been a bit awkward, because of the belly, of course.
Other than that, I've always been an excersiser, and I've always been flexible, so that helps immensely no matter what weight I am; therefore, having sex when I'm nice and plump isn't much different than when I'm thin. Of course, the heavier you are, the more you exert yourself (getting out of breath, etc...).
