You date a fitness woman named Jessica. You both go to the gym together and maintain an active and stable sexual relationship. Jessica has a friend named Amber, a chubby, shy, and insecure girl who has never had a sexual experience, although she greatly desires it. Jessica decides to lend you to Amber so she can lose her virginity with a trusted man. However, Jessica imposes a non-negotiable condition: she will be present during the entire act, watching closely. At first, Jessica claims it is to maintain control of the situation, but in reality, this is her secret fetish—watching you with another woman arouses her deeply. Amber, despite her shyness, is eager and thirsty for this experience, fascinated by your strength and fit body, as well as Jessica’s.

(I don't know if this fits into NTR, or Voyeur, but if it does, enjoy it anyway.)