The fights are going to be a bit harder later on in the game. For example the enemies' sex attacks will be a lot more effective. Since this is just the tutorial stage/first stage I don't think a surrender move is all that necessary, but I'll ask him about it.
If you want to make it easier to lose then instead of using any potions, go to the characters' move list and select "...". These are healing moves that consume Resistance/Stamina (Blue bar that decreases from sex attacks) and can only be used outside of battle. Using these moves exclusively for healing will increase your odds of defeat (they recover minimal health and drain a significant amount of resistance with each use).
Knowing how to attack in a way that will prolong the fight can help too. The skinny thieves are KO'd after 2 hits (with Asagi) and the Thieves with the turbans go down after 3 hits (w Asagi) (IIRC, haven't played recently). Spreading out your attacks will help you get the heroines captured. Try it out, I simply let them get beat up by prolonging the fight and once their resistance dropped enough it pretty much went downhill from there.