Basic Trope: A man proposes marriage to a pregnant woman to protect her reputation.
Straight: Alice finds out she's pregnant, but has no idea who the father is. Bob agrees to marry her.
Exaggerated: Bob meets Alice (six months pregnant) for the first time on the bus. He sees that she has no ring, and proposes to her right then and there.
Bob is the father, and he feels that the honorable thing to do is to be there for Alice and his child, so he asks Alice to marry him.
Alice is in a relationship with Bob, and one day she is raped on her way home from work. She finds out she is pregnant, and Bob (already supportive of her) asks her to marry him, so he can be there for her all the time.
Single motherhood is hard. Alice is a dear friend, and Bob just wants to make sure she'll have someone around to help take care of the baby and bring in money.
Alice can't afford health insurance, and is worried about what would happen if she and her child were to get sick. Her close friend Bob works a job with great insurance for its employees... as well as their spouse and spouse's children.
Lampshaded: "Oh, Bob... That's very sweet, but we're not in some 19th century Irish village."
Invoked: Alice fakes a pregnancy, and spins a heartbreaking sob-story around it, in order to convince Nice Guy Bob to marry her.
Bob is afraid of what others will think of him for taking "damaged goods" as his wife, so he does nothing.
Alternatively, Alice turns down the proposal. (She doesn't know Bob well enough, she doesn't find him attractive, she doesn't like him, she doesn't feel like she needs a man to provide for her or the baby, she wants to Marry for Love, etc.)
Bob: I do love you, Alice, but... Platonically, you know? Alice: Hmm... Would you marry me if we're both single when we're forty? Bob:[laughing] No. Alice: What if some heartless cad gets me pregnant and runs off in the night, leaving me raising a baby alone with crops to harvest and bandits to fight off? Bob: It's a deal.
Conversed: "Ugh, she's not really going to marry that loser, is she?" "Well, we may not like it, but in the world they live in it's realistically her only chance for a decent life..."
Deconstructed: This has a lot of Values Dissonance; in some more traditional cultures, this is a way of protecting a woman's reputation and ensuring that someone will be able to support her and the baby (see "Justified"). In other societies where sex and babies outside of marriage are more or less accepted, this is seen as anachronistic and sexist.
Reconstructed: Alice marries Bob because she loves him, not because she's knocked up and needs a provider.