I believe her based on his being a general liar and her relating the story sounds credible.
The issue is the cover up, the payment to keep the American voters from learning about something (this and other sexual exploits, for example) before the 2016 election and doing this via election fraud (payments that exceed the allowed amount and not properly disclosed) and false reporting of payments in his business records (retainer and legal fees paid to the person responsible for covering this up).
The thing he was trying to cover up may have been titillating, but it wouldn't matter if he had wanted to cover up eating extra ice cream (A Biden possibility), underpaying his staff (a Trump probability), or shooting dogs in the face (A Noem admission).
