Continuity: We find out in this episode that Carol told Ross she was a lesbian one year before the Season 1 pilot episode. Yet at the end of Season 1, almost two years later, their son Ben is born. Ross and Carol must have had sex a year after they separated and while Carol was with Susan.
Note: Whenever it was that Carol realized she was a lesbian, we know that she and Ross didn't actually separate until the start of Season 1, so she wasn't "with Susan" at the time Ben was conceived. Carol discovers she is pregnant soon after she and Ross separate. In Season 9's "The One with the Boob Job" when Ross is describing his and Carol's marriage to Phoebe, he says that they had some good times before Carol became a lesbian and "once afterwards". This "once afterwards" likely refers to the time Ben was conceived.