In the MCU Nat doesn't really have a love interest, most of the time.She flirts with Cap in Captain America: The Winter Soldier and they kiss but that's just to avoid drawing attention to themselves.
Later on we see she's close to Hawkeye, however, that is a close friend friendship and not a love interest. She's even the only Avenger to know about Clint's family and safe house. So much so that they were naming their newborn after her.
Lastly, she's close to Bruce as we see in Avengers: Age of Ultron and there could actually have been a potential for a couple between them. The both liked each other but Bruce was scared to make the leap into the relationship. All chances of redeeming that were killed off though as Hulk left Earth at the end of Avengers: Age of Ultron as we find out in Thor: Ragnarok. He's gone for so long that Nat moves on and it's "awkward" when they finally meet again in Avengers: Infinity War. However, it's clear from their interaction in the film they've moved on from each other.
Natasha Romanoff: Hi, Bruce.
Bruce Banner: Nat.
Sam Wilson: [Whispering] This is awkward.
Avengers: Infinity War