I think the idea of him seeing a sex therapist is a great idea.
In fact I think anyone with a particular problem seeing an appropriate professional for help is a great idea.
What I am NOT seeing as such a great idea is you "marking time" while he undergoes this help. Particularly as his commitment level to the therapy is not known, nor is his prognosis.
You could easily trash another year or two whilst he "tries", you could easily then trash another year or two as he "succeeds" of "fails". And there you'll be 4 or 5 more years down the track confronting the exact same issue and choices you face today, but having pissed another half decade up against the wall.
Here's an alternate scenario for you.
Off he goes to the sex therapy. Works hard and diligently, or just dicks about wasting time.
You continue formulating your exit strategy and knocking it into do-able shape, and continue to sort your own shit out.
As soon as you can, you have a (cough cough) "trial" separation. During which you keep working on your shit unhindered by his presence. And he works on his shit unhindered by your presence.
At the conclusion of this "trial" separation - or at a time you nominate - you make the call to reunite or turn the trial separation into permanency.
This scenario would be *fair* on everyone.
It keeps you moving forward.
It gives him the time he wants.
Don't make the tactical error I made back in the day.
At one point I confronted my spouse saying words to the effect - "I know you are struggling with some issues, and I am here for you while you sort them out, but once they are sorted out we are going to have to decide whether to wind this up or not. But until then, I am here for you".
That was very noble and thoughtful of me don't you think ?
WRONG !!!!!!
What I had done was provide a powerful incentive for her NOT to try and sort her shit out, because I had committed to staying until such time as she sorted her shit out.
Run a clock on it. Don't give open ended committments.