In the office (US) Dwight and Angela had a secret at first relationship which was later revealed to the entire office. While they were shown to be greatly involved in their 'romance' it was never shown how and when did they actually start.
in S02E02 Sexual Harassment, Dwight reaches out Toby to ask some very awkward questions. Questions he would not have asked if had been involved with a women sexually. So I infer he wasn't at this point.
But then, in S02E08 Performance Review, Jim argues Dwight that You are not having sex. To which Dwight gives out a smirk as if he knows Jim is not true. I infer Dwight started with Angela by now.
And in the very next episode Email Surveillance, Dwight and Angela are at it in Jim's House party! So it is confirmed they are involved by now.
Considering Dwight's quirks and Angela such uptight personality traits, when and how did they actually start their affair? How did they even end up together! was it ever implied or inferred?