@letitbleed you can go through that and still ABSTAIN from any sexual acts because you are so conditioned to it. this is why so many girls in the 1800's were so afraid that they were going to hell because of their strict upbringing and every month they would have a period. To them they were told this was not pure, this was potential sin so they thought they were dying and finally they realized they wouldn't die, but it was something they had to live with. Now Just because they went through that doesn't mean they become sexual beasts, but if conditioned right they will follow the religious path and not do anything even though they have "urges" as they are taught they can over come the sinful urges.
Now Elena comes along and starts to liberate D's mind and that is then cemented when she reaches F to find he's not the big ogre that the mother made him out to be. so that is why D is opening up to him faster than one might think

you have to take all this into account when you try to make this game into more than it is and say it's not being treated with any sembelance of reality.
@Kryptozoid you can survive going to age 18 without sexual contact. it's been done before, it's been done in today's day and age. some people just have a greater will to keep them from doing it than other. it's plausible that she is 18 just as you said it's plausible that this might be based upon someone younger. the urges are there but the upgrining wins out till the strictness is no longer there to continue.
@RobJoy my appologies i had the wrong person tagged in this