I answered on Hex's thread, but basically in the normal mode you can only get MP restoring potions from men (as a certain bodily fluid) and unlocking some lesbian sex moves was tied to straight sex progression (e.g. footjobs unlocked foot worship, that sort of thing). In yuri mode, all lesbian sex movies are unlockable via lesbian progression and you can gain MP restoratives from either men or women. Basically the net effect is it makes straight sex optional without needing to make any sacrifices.