Jane Austen was acutely aware of human nature/human personality, and she wrote her heroines as human beings. They all have their imperfections and I think that Austen was very knowledgeable about the effects of persuasion. For example, Anne Elliot allowed herself to be persuaded not to marry Wentworth.

In the same way, Marianne might be persuaded by Willoughby to (I'm trying to think of a more delicate way to put this) go to bed with him. As has been pointed out, not all the girls who were seduced by men were bad, immoral girls.

Maggie Lane writes that behaviour between young couples which was prohibited before marriage included:

"...using Christian names, unless connected by family; driving in carriages alone together, correspondence, exchanging gifts, and any kind of intimate touching. If any of these were observed to take place, then the automatic assumption was that the couple were engaged."

Marianne and Willoughby do all of these things, which meant overstepping the rules of society and propriety in the Regency era. I am not implying that Marianne is without principles and sense but she delights in defying convention, as we see in her affair with Willoughby. She does not pay any attention to society etiquette, e.g. being chaperoned.

"Marianne and Willoughby are guilty of all five kinds of transgression. He calls her Marianne; they drive to Allenham alone on the day the picnic is cancelled; she writes to him in London; he offers to give her a horse, which she enthusiastically accepts; and she allows him to cut off a lock of her hair. The catalogue of these transgressions is enough to indicate how close her unconventionality brings her to losing her honour"

(Quotes in italics from Jane Austen's World: The life and times of England's most popular author by Maggie Lane.

I am not suggesting that Austen intended her heroines to be seduced but the danger is definitely there - if Marianne had been alone and parentless, like Eliza, I think that it would have been a possibility for Willoughby to seduce her.

I was reading Jane Austen: A Life by Claire Tomalin yesterday, and she says the same thing: "[Marianne] follows the dictates and desires of her heart into unconventional and even risky behaviour, justifying it with the words 'we always know when we are acting wrong'...she might have suffered the same fate as Colonel Brandon's niece, Eliza Williams, seduced by Willoughby and left pregnant" (Claire Tomalin, Jane Austen: A Life, pp. 158-159).

Willoughby seduced Eliza and jilted Marianne - not very good behaviour, IMO. What Elinor says in the clip linked below explains my feelings exactly:

"Willoughby: My aunt had somehow been informed of an event, an affair, a connection. No doubt you've heard the story.

Elinor: I have. A child, an innocent girl, only fifteen years old, whom you abandoned without a thought"

Willoughby: "And because I was a libertine, she must be a saint, I suppose...[pause]... I don't mean to justify myself"

Elinor: "And what did your aunt say?"

Willoughby: "[angrily] I was dismissed from her favour, and from her house. I was virtually penniless, in debt and without any prospects"

Elinor: "So you set off to London to find yourself a rich wife"

Willoughby: "What could I have done?"

Elinor: "You could have made amends to that poor girl you seduced, and you could have told my sister the truth"

Quite right, Elinor. Willoughby could have and should have made amends to Eliza.

Quotes above from S&S3, clip of episode 3