Maybe a space station laboratory with a Mad Scientist who's trying out their new mind/body swap device.
It starts off with an Extranet Mail wanting to call Captain Steele over to the lab to either a new piece of tech to be tested, or a cry for help. Steele is lured in, gets captured, and then swapped with a test subject like an alien animal. Their old body is lured away, and they are then put into a cage to await further testing. Steele escapes captivity and learns their new anatomy, and fights/fucks his way to defeating minibosses (lab assistants, security guards, robots, other test subjects) and either stays or swaps bodies with them to get better combat stats, or seduction stats. Eventually, Steele finds that the Mad Scientist has swapped bodies so that they're now in Steele's body (wanting to become more sexy/stronger/etc), and the final boss fight commences. Upon defeating them, you can literally fuck yourself, and have access to the bodyswap device in the lab for more sex scenes as a reward. By the end of the mission, you're back to normal.
Lots of potential for sex scenes with different bodies, and bad ends (stuck in a body, or being fucked by your old body).