there are no documented case -- ever -- of a sober mother rolling on to her baby. my daughter slept between me and the wall so that we didn't have to worry about her dad.
sex! come on now! :) we have sex plenty of places other than the bed. she napped in her crib, and we kept her in a sleep positioner if she was in bed without us.
we moved her to her crib at 9 months because she, and hence i, could no longer sleep in our bed. she woke every hour all night long for 3 months (oh yeah). she was one of the few babies for whom co-sleeping was eventually not the best arrangement. but we miss it and would go back to it in a second if she could sleep that way (she sleeps soundly in her crib now).
WHY? oh jeez, why not? i wanted her to be close to us at night when she was first born. i never felt right having her sleep alone. i felt -- and i am not judging those who don't, just answering your question -- that it was unnatural to put a little one, so new to the world, in a crib alone. and then we realized how much we loved sleeping with her and didn't want to stop. furthermore, it made night feedings so much easier -- both breast and after we had to switch to formula. i could dump the pre-measured water into the bottle of pre-measured powder and get to her before she even fully woke. so she and i barely woke, she ate, and were right back to sleep.
i loved it, it (until a point) was the best way ot maximize sleep, and, best, it gave my baby extra security and bonding in her earliest days while also reducing the risk of SIDS (which it is proven to do). i never once worried about rolling on her.