Well it doesn't really come up. With the exception of once I have an ex that is just fantastic in the sac and yeah I bought up the "but we can still do it" conversation. Mind you we still do and that is been going on for over 5 years after the fact.
So to answer your question I think it just depends on the couple. And if you are the "other woman" why would he not still have sex with her if she so wanted? I do not think that he has any moral problems with cheating on his ladies.
In my opinion, if a couple is separating, they are agreeing to living separate lives. Unless they are separating with the intent to work on their marriage and get back together, their sexual activities are private.