At the moment sex acts are focused on 'initiating' body parts and 'receiving' parts - ie you can do things to an ass, but not with an ass (for the currently implemented actions). I probably am going to need to implement some sort of 'I request that you do something to my ass' system so you don't need to wait for the NPC to get the idea randomly.
As for crew, once the gameplay loop is implemented and there's crew requisitions and the station shop, you'll be able to exchange crew members after the tutorial is over.