soap is a chemical base, the reason why bases are slippery is they remove the top layer of skin. Soap is less of a base but still alkali. Really, if you are worried, then you should just rinse the soap under water.
I am assuming psychologically you are equating rubbing the bar of soap to your genitals to rubbing his used underwear as well. But that is not why soap is used.
It gets rid of oils produced normally in the skin, sebaceous glands and the like. When the oils, especially in the sweat glands in the armpits and genital region (they produce an oilier mixture than watery cooling function say from your forehead and bacteria feast on that and create a smell). What soap does is emulsify that and you can rinse it away removing smelly bacteria colonies.
Soaps are useful for cleaning because soap molecules attach readily to both nonpolar molecules (such as grease or oil) and polar molecules (such as water). Although grease will normally adhere to skin or clothing, the soap molecules can attach to it as a "handle" and make it easier to rinse away.
(fatty end) :CH3-(CH2)n - COONa: (water soluble end)
The hydrocarbon ("fatty") portion dissolves dirt and oils, while the ionic end makes it soluble in water. Therefore, it allows water to remove normally-insoluble matter by emulsification.
So any ball or armpit oil still on the surface of your bar of soap, can easily be removed by rinsing it in water. I dont have roommates, but if I did, I'd be more concerned about all the dead skin flakes they shed everyday which breeed the ugly dust mites that infest your bed. Basically you are sleeping with both your roommate, and the devil spawn that your roommates skin feeds.