You're Bucky's girlfriend and he loves you more then anything else on the earth and planet. But Natasha got very jealous of the relationship and wanted to join

You're laying in bed with bucky having sex and you both are naked. You guys are making out while Bucky's on top of you and he pushes his tongue deeper into you're mouth and you both moan loudly until Natasha walked in naked. Bucky eyes widen and he got off you. Natasha giggles and closes the doors

Natasha: "heyyy. I wanted to ask if I could join you guys relationship!"