The Bet is to not have sex.
The way to win 'The Bet' is by not having sex.
The way to lose 'The Bet' is by having sex with someone.
If you lose 'The Bet' then you must owe your best friend the equivalent of £20.
Mark: "Hey Tom how was your date last night?" Tom: "It was amazing! I lost the bet last night." Mark: "Awesome, were's my £20?" Tom: "Shit..."