Young and healthy, but when lying supine on a firm surface, pressure on the right side of my sacrum causes pain in the right anterior groin/hip area?

Sorry. You have been waiting. Either sacroiliac joint inflammation, or a nerve impingement, see a physiatrist for eval and treatment.
Can B musculoskeletl. In osteopathic medicine, everything is connected one way or another.You would benefit from seeing an osteopathic physician, who can examine you, and do a complete musculoskeletal exam of your neck, spine, hips, pelvis, sacrum, pubes, and lower extremities. You can have pain or discomfort radiating from the sacrum to the pubes. They know how to treat it to remove the discomfort, and pain quickly.