As someone who has served on a US submarine, I have to say that honestly I don't think that will be happening anytime soon. And not for any PC or non-PC reasons, but really for the simple reason of logistics.
The US Navy's current policy is that women and men must have separate and berthing and heads. The average fast attack submarine (which makes up the majority of the fleet) has only 2 heads, one with 3 stalls, a urinal and a shower. The other has only one stall and one shower. All of this has be shared between 120 people. With just men there is already a wait just to be able to use the bathroom on a regular basis, women would only make this worse. Meanwhile, the smallest berthing space has 9 racks and those are typically all shared. (3 people for every one rack)
Until they change the design of submarines (something that would cost billions of dollars) I don't realistically see a change in the current male-only policy for submarines.
To be quite honest, if they were to try and force a co-ed force with the current sub designs I would feel incredibly sorry for whatever women chose to volunteer first. They would probably be victim to a tremendous amount of hazing not just because of their sex, but because of the preferential treatment they would receive. (i.e. 'Why should this NUB girl get her own rack when I have to share one and have been here for years?'
I won't even go into the other types of drama that runs rampant on ships with more living space, but I'm sure that would also be an issue.
Bottomline is that current logistics and policy can't support a co-ed sub fleet.

ET1(SS) Grimbear