I have played all routes so let me of some assistance!
Spencer: You can top and bottom depending on confidence scores (to be on bottom choose choices that are not confident)
Harold: You can only top
Dozer: You can only top
Darius: You can only top
Chester: You can only bottom (atm, if you choose you want to top Chester, you actually skip the sex scene, so I am incliined to believe that eventually all the routes will have both options otherwise this route wouldn't show the option if all it does is skip the scene)