Why is it a problem to have women on a submarine?

The living quarters on submarines are very, very small and it is difficult/impossible to give women their own space (including bathrooms, dressing rooms and bed rooms). Women would have to sleep in the same bunkbeds as the men, dress in front of them and see men dress.

There is an added danger of sexual harrasment because there is no privacy-- and any potential emotional tanglements could be pretty bad for the operation of the submarine. If you are deep under the surface of the ocean, there is no place for anyone to get away.

However, the main reason is the cramped living spaces, where women would not be able to have their own quarters.

It is a mater of society. The US does not allow it as for the most part we are prudes that cannot handle being around the opposite sex without difficulty. The Norwegian, Swedish, and several other navies have coed crews on even smaller subs than the US uses, and there are no problems with it. The lack of quarters is just an excuse. It is true, but it can be overcome. The crews normally have to share racks (beds) with other crew members. Three watch rotations equals three people rotating out of the same rack with only a change of linens in between.