It has nothing to do with probability whatsoever. Whether you are at "fault" for a situation, and whether you just did something that had a chance of leading to that situation (whether it is a high chance or a low chance) are two separate things.
For example, if I am in a car accident, I chose to get into the car, and I wouldn't have been in the accident had I not done so. But the fault of the accident is not necessarily mine, it lies with whoever was driving their car in an inappropriate manner. Similarly, if someone kidnaps me and forces me to donate my organs, I would not be in that situation had I not been walking around in the area where the kidnapper happened to have caught me. I did something that had a chance of leading to that situation. But the fault lies entirely with that kidnapper, and not at all with me. In both these situations, the fault lies with a person who acted unreasonably i.e. the reckless driver and the kidnapper. The individual suffers as a result of someone else doing something that they should not be doing, thereby forcing the individual into an undesirable situation.
The same cannot be said of pregnancy (unless it's a case of rape or something like that). There is no analogous equivalent of the reckless driver or the kidnapper who can have the blame for the undesirable situation placed upon them. So there is no "fault" as such. The couple can't blame someone else for acting in an inappropriate manner that resulted in a pregnancy. The pregnancy is a result of nobody's actions but their own.
There is a fundamental difference between going for a walk outside at night down a dark alleyway and getting killed by a gunman, and going for a walk outside in a lightning storm and getting killed by lightning. In the former situation, you can blame someone else for acting inappropriately (as you can with some car accidents, and with forced organ donation). In the latter situation, you can't blame anyone else - you just took a risk with nature and fell foul of it. The probability of the event occurring has nothing to do with it.