The Honeymoon Killers

Martha Beck is a sullen, overweight nursing administrator living in Mobile, Alabama, with her elderly mother (played by Dortha Duckworth).[4] Martha's friend Bunny (played by Doris Roberts) surreptitiously submits Martha's name to a "lonely hearts" club, which results in a letter from Raymond Fernandez of New York City. The audience sees Ray surrounded by the photographs of his previous conquests as he composes his first letter to Martha. Overcoming her initial reluctance, Martha corresponds with Ray and becomes attached to him. He visits Martha and seduces her. Thereafter, having secured a loan from her, Ray sends Martha a Dear Jane letter, and Martha enlists Bunny's aid to call him with the (false) news that she has attempted suicide.
Ray allows Martha to visit him in New York, where he reveals he is a con man who makes his living by seducing and then swindling lonely women. Martha is unswayed by this revelation. At Ray's directive, and so she can live with him, Martha installs her mother in a nursing home. Martha's embittered mother disowns her for abandoning her. Martha insists on accompanying Ray at his "work". Woman after woman accepts the attentions of this suitor who goes courting while always within sight of his "sister". Ray promises Martha he will never sleep with any of the other women, but complicates his promise by marrying pregnant Myrtle Young (played by Marilyn Chris). After Young aggressively attempts to bed the bridegroom, Martha gives her a dose of pills, and the two put the drugged woman on a bus. Her death thereafter escapes immediate suspicion.
The swindlers move on to their next target, and after catching Ray in a compromising position with the woman, Martha attempts to drown herself. To placate her, Ray rents a house in Valley Stream, a suburb of New York City. He becomes engaged to the elderly Janet Fay of Albany (Mary Jane Higby) and takes her to the house he shares with Martha. Janet gives Ray a check for $10,000, but then becomes suspicious of the two. When Janet tries to contact her family, Martha and Ray hit her in the head with a hammer and strangle her to death. They bury her body beneath their cellar floor in her trunk, tossing into the grave's dirt the two framed depictions of Jesus that, Martha notes sarcastically, she'd told them she took everywhere she went.
Next, they spend several weeks living in Michigan with the widowed Delphine Downing and her young daughter. Delphine, younger and prettier than most of Ray's conquests, confides in Martha, hoping that she will help her persuade Ray to marry her as soon as possible because she is pregnant with Ray's child. Martha is in the midst of drugging Delphine when the woman's daughter enters the room with Ray. He shoots Delphine in the head, and Martha, off camera, drowns the daughter in the cellar. Ray tells Martha that he must proceed with his plan to move on to one more woman, this one in New Orleans, and then he will marry Martha; he reaffirms his promise never to betray her with one of his marks. Realizing that Ray will never stop lying to her, Martha calls the police and waits calmly for them to arrive.
The epilogue takes place four months later, with Martha and Ray in jail. As she leaves the cellblock for the first day of their trial, Martha receives a letter from Ray in which he tells her that, despite everything, she is the only woman he ever loved. Titles on the screen then conclude the story, saying that Martha Beck and Raymond Fernandez were executed at Sing Sing on March 8, 1951.[5]

Question: who promises Martha he will never sleep with any of the other women, but complicates his promise by marrying pregnant Myrtle Young?
Answer: ray