This is sort of similar to some other prompts, only with Nat instead of Bucky.

So Natasha underwent lots of sexual abuse while she was with the red room / KGB, and now as a free person, finds that she's developed a bit of a fantasy about being raped and approaches Sam, Steve, Bucky (and/or others if you prefer) about fulfilling the fantasy for her. She's not otherwise romantically or sexually involved with them, it's more about their strength and the fact that she trusts them to stop if she does safeword out.

I imagine it's very violent (hair pulling, breast torture, spanking, facefucking, double/triple penetration, bondage) but really gets Natasha off.