If you ask most people what their favorite way to work out is, a lot might say it's with a partner. But what they may really mean, is to have sex. Depending which position you utilize will vary which area you get to workout. Missionary is a great workout for your thighs and butt. If the person on the bottom focuses on engaging their core to thrust instead of using the lower back to get propulsion, it will strengthen the abs Doggy style can be a great upper arm workout for the person on the receiving end. Modifications like placing the hands up against the wall to help thrust would be a good shoulder and arm workout.