"Tales from the Darkside" (TV anthology series) Season 3, Episode 6
episode: "Black Widows" (Nov. 2, 1986)
From IMDb review:
In this episode, we meet a poor girl and her mom living in a trailer park. The girl wants to get married, but her mother never approves. When she does get married, she learns the secret of her family and why she never met her father: She's a black widow spider (and I suppose a Black Widow). Once she marries him and consummates her love, she kills him but has a pregnant stomach to show for it.