you lose the muscle mass due to the lack of stress on the system - that is why the workout while they are up there. and the lack of gravity (or the very low gravity) also makes their bones weaker b/c they need a stress on them to retain their density.
In a few years, hopefully within the century - they'll have a spinning space station with artificial gravity, then it will be a lot easier to work in space, and have a weightless area in the center.
there is much debate as to whether sperm can fertilize an egg in space and whether or not gravity or the lack thereof will fuck up the orientation and where they travel.
and spiders can't weave their webs correctly without gravity.
and bees don't build their honeycombs right.