He's separated...has Filed...the legalities of Sex

Stay friends for now and wait til he is divorced. Make sure before you take that next step that you are not a rebound for him.
Note too- I speak from experience here- the stbx will not get any nicer. Do they have kids? How is all of that handled? Realize that if there are kids- she will always be a part of his and your life. That will NEVER change. You can't fix it- you can't change it- Make SURE that things are settled- meaning visitation etc before you commit. Also note- don't spend overnights with the kids around- if there are kids involved- it can hurt things legally for him.
In other words- he may seem like Mr Wonderful , and he may actually BE Mr Wonderful, but you need to keep your eyes open and know that he had a part in the reason for the divorce. It takes two to tango. His part may be minimal but I am sure you will find that it was partially his fault also. He will blame her for everything , I promise.
Just be careful honey> I know first hand how a vindictive ex can control a relationship - and it is not a picnic!

In my state the answer is yes point blank.  If you have sex while you are married it is adultry.  It doesn't matter if you are separated pending divorce or legally separated (two different status in my state).  It all depends on your state.

Well, technically, if you're having sexual relations and you're still married, it's adultery...the only difference is if you're legally separated, there may or may not be any legal ramifications for it...again, it all depends on the state, how long you've been married, and a lot of other factors.  That would be the short answer.

Basically would having a sexual relationship be considered adultery?

Thank you. His state has added irreconcilable differences as a "no-fault" grounds.  He does have kids.

I had wondered what the emotional process is.  He's wanted the process to move along before I came on the scene.  I had wondered though, what the emotional layers of the final dissolution might be to unfold.  He's already felt "like a failure" as he's described to me.  

Morally and legally. For legal go to the white box in the upper right hand corner: state divorce laws and calculators.

You don't say what state you are in. He has a legal separation so that should cover the fault based southern states. As general info if you are in a fault based state any sexual relationship could ffect his property settlement depending on the state. Does he have children; that could affect every aspect of a new relationship. His stbx could focus her anger on you and use his children as punishment. It happens sometimes.

Morally that is up to you. Everyone has their own moral compass. I agree with paula and angielou. Wait until the divorce is over and the dust has settled. You really don't want to be a rebound relationship. If he cares that much he will wait. He needs time to work on his issues left from their marriage before he comitts to a new woman in his life