The case literally has zero to do with an affair. The alleged affair could be a complete fabrication. The point of the case is that Trump directed his lawyer to use money for XYZ. Trump then put that money down on the ledger as ABC. That he claimed it was ABC and not XYZ is the entire point of the case.
The WHY he decided to commit fraud is a nice detail but literally has zero bearing on if Trump actually did the whole I said the money was for ABC but it was really for XYZ.