I’m sorry if I’m just being too much of a dodo to perceive the mystery, but your scenario seems easily accounted for. You can use a Bayesian network to infer causality if and only if you have valid data to fill it with. Of course wearing large pants does not cause one not to exercise, but no real set of data would indicate that it did. Am I missing something?
EDIT: shortly after writing this, I read up on faithfulness and Milton Friedman’s thermostat, so the “if and only if” part of my comment isn’t quite accurate. Still, the pants size scenario doesn’t seem like one of these exceptional cases.
I would be completely on-board with this if there was a method of improvement other than IVF embryo selection, since I consider human embryos to have moral value. Even if you don’t, unless you’re very sure of your position, I’d ask you to reconsider on the basis of the precautionary principle alone—i.e. if you’re wrong, then you’d be creating a huge problem.