I've created a position I call the "half missionary" for those with bad backs.

After fucking people for years on my massage table, I've found that many people can't put their legs on my shoulders when I fuck or even pull their legs up to their chest without a lot of effort. Early in my massage practice, I worked on a lot of heavy people and people with bad backs and little flexibility so I developed a technique that actually allows me to slide completely into them (balls deep) and still get my nipples worked while I breed a hole or pussy. You put the fuckee on his or her side with a pillow between their knees. Get them to form a "C" shape with their bodies with their ass slightly hanging over the edge of the bed or massage table. If they can twist their torso a bit to face you, they can stiil reach up and twist your nipples. I originally learned this from working on big bears with bellies and very pregnant women. You will be amazed how much of your cock you can get inside someone like this. It also rubs the side sections of a vagina or butt that are often neglected in a regular missionary fuck.