Regardless of the couples inability to actually have sex, they should still refrain from co-habitating because of the potential for scandal if they are faithful Catholics. I have seen faithful catholics come on these very forums and ask whether it would be ok for them and their spouse to be to live in the same house even before they married when they are both committed to purity, and the answer was invariably no, because in spite of their good intentions the potential scandal is still there (not to mention occasion for sin) and that is enough to dissuade them from living together before marriage.
And the intentions of these moral premises is not to cause grief, and sadly stuff like this happens all the time and everyone has their crosses to bear. The couple that is married and desperately wants children cannot conceive, a young wife is abandoned by her husband and cannot marry again and has no choice but to live alone, and those who cannot physically have sex cannot engage in marriage because that is what marriage itself entails. I hope I am not being curt because that is not the intention, but if we know the Truth we need to live by the Truth and sometimes it hurts… heck it may hurt all the time…in this life. but pain and heartache in this life is a small price to pay for the reward in the next life, and I am thoroughly convinced that those who faithfully bear such difficult crosses in this life will be rewarded immeasurably in the next.