It was previously reported that Steve Rogers’ love interest was Betsy Ross but Deadline is reporting the love interest is Peggy Carter:
UPDATE: Marvel Studios is also testing leads for the female role of Peggy Carter. While Marvel is looking for an American actor to play Captain America, the candidates for the female lead are British. So far, Keira Knightley, Alice Eve and Emily Blunt are under consideration. Blunt is known to be a favorite of Marvel brass. She was offered the role of Black Widow in Iron Man 2, until Fox exercised an option it held on the actress and put her into Gulliver’s Travels. Scarlett Johansson took the role.
More info about the love interest:
The female role is that of Peggy Carter, a girlfriend of Captain America during World War II. She was also an American agent serving with the French Resistance. She has standard military training and is handy with a gun. She is also a competent spy and detective. Her niece Sharon Carter becomes a prominent figure later on in Captain’s life as she works for The Avengers and occasionally dates Steve Rogers.