Why does Stella marry Stanley in A Streetcar Named Desire?

The background to Stella and Stanley's marriage is not explained directly in the play, but Stella's reasons for marrying Stanley are implied.

It is implied in the play that Stella met Stanley in New Orleans, where she moved from Belle Reve (the plantation in the countryside where she and her sister Blanche were raised). Stanley, a strong, gritty Polish immigrant, would have been different from the other men Stella knew. Blanche suggests that Stella was seduced by Stanley and that she is enamored with his masculinity. 

Stanley, however, suggests that Stella married him because she wanted to get away from the quiet, well-ordered life of Belle Reve. He says, "I pulled you down off them columns and how you loved it, having them colored lights going!" In this passage, the "columns" refer to the stately columns of Belle Reve plantation, which represents Stella's rarified social status. According to Stanley, Stella was excited by the "colored lights" of New Orleans and the life he gave her there.