Intimacy is important, and usually involves quite a lot of close contact. On earth that contact is held in place (literally) by gravity. Horizontal or vertical, your body is affected by gravity, and the friction that exists between the bed and your feet (or hands, or knees, or face, or pinky, whichever organ you use to balance yourself while you're in the mood for dancin') keeps you from moving anywhere other than where you intend to.  In space, however, things are much different.