Being bisexual is not about being in boy-boy-girl threesomes.
On the other hand, being bisexual isn’t about not being in boy-boy-girl threesomes.
Nobody in this scene is named Pierre. “Lucky Pierre” is an old slang term for the middle person in a threeway, fortunate enough to fuck and be fucked at the same time. Jasons friends here were Tanya – a straight girl with an understandably healthy appetite for men – and her boyfriend Michael – whose appetite was mostly for women, but was certainly enthusiastic enough about doing it by proxy.