Edit: the following is wrong, Fuck Vampire in particular just has a minimum flux cost much larger than other Fuck actions. That might be a bug?
The flux costs of at least the first iteration of some actions (I've noticed this most prominently on Fuck Vampire but I expect it happens in some other places) seems to be determined on entering the area rather than when you take them. In particular, Do Makeup does not reduce the flux cost of an immediate Fuck Vampire, though it does reduce the flux cost of further Do Makeup actions in the same chain. (I'm not sure whether this is true of you add the Fuck Vampire to the action list after starting the Do Makeups but I believe it is.)