He thinks (has thought all along) that marriage to Jane will get him out of his moral fix because she's innocent, unspoiled and if he sticks his dick in her some of that innocence is bound to rub off. Men have been thinking this of women since forever and it's the most awful shit and Jane- though he's subjecting her to a frightful emotional mauling- is having none of it.
How did Bronte get to know so much about men?