If you are on the bottom:
For the one on the bottom, the missionary position can be modified so that one leg is bent up, resting on the outside of your partner's leg. Another possibility is to sit at the edge of a chair with the leg on the painful side up so that the heel can be placed on the seat of the chair. The other foot is placed on the floor. The top partner kneels on the floor and enters from the front.
If you are on the top:
The missionary position can be modified for the top partner who has pain on one side by propping the bottom partner up with pillows. That way you can lay on top with your leg on the painful side bent.
This option is a good one for developing emotional intimacy. Both partners can lie on their sides, with one facing the other. If you are on top, place a bent leg under your partner's (closest) leg, which should be bent.
Hebert, Lauren, A., P.T. (2001). Sex and Back Pain. Greenville, ME: IMPACC USA.
White, A., III, M.D. (1990). Your Aching Back: A Doctor's Guide to Relief. New York, NY: Simon & Schuster/Fireside.
Kisner, C., & Colby, L.A. (2002). Therapeutic Exercise: Foundations and Techniques.Philadelphia: F.A. Davis Company.