It’s one thing a taboo and another thing is emotional recoil, so to speak, how a person feels after he violated his taboo. If he hates, most likely does not violate the taboo. If he does not like, then the taboo will break, but then he will feel unhappy. and in third is this locked by the plot - it is need to be unlock by the plot (by event).
but True, some types of sex, even if they are blocked in the plot, can be carried out through the correct sequence of actions. For example, vaginal sex with a cumming-in during morning waking up after a joint sleep in the same bed. True, this is inaccurately, that is, is there a chance for a girl to get pregnant as a result of such a creampie, it is unknown to me for certain, but perhaps a small chance but it is. Although "officially" a girl cannot be persuaded to vaginal sex when she is awake - until vaginal sex is unlocked by the event by the plot.