A bounder and a cad!

For interested persons:

Here is some information about Regency romance, which is useful background when reading Sense and Sensibility, Pride and Prejudice, etc.

If there was an engagement (or a publicly perceived understanding, or obvious display of affection) between a man and a woman, then the man was bound to marry the woman at all costs.

"Has not his behaviour to Marianne and to all of us, for at least the past fortnight, declared that he loved and considered her as his future wife...?"
Sense and Sensibility Mrs Dashwood, Chapter 15.

If he broke off the engagement or tried to back away, his rejection of the lady was seen as a slur against her reputation and her family (was there something wrong with her? was she less than pure?). An insult to reputation and family was disastrous - it might ruin her future prospects for marriage, and the respectability of her family. Therefore, a proper gentleman was bound by honour to his commitment (or a perceived commitment) to the lady. If a man acts the cad and dumps the lady, he could be called out to duel with her father, and settle the matter in public.

A gentleman would always act with honour towards the lady, even if it makes him unhappy: for example, Edward Ferrars gets engaged to the horrid Lucy in his foolish youth; Capt Wentworth incautiously spends too much time with Louisa and everyone thinks they are in love. Neither man would act dishonourably and break it off.

It is ONLY EVER the lady's prerogative to end an engagement and release the man. Fortunately Edward loses his fortune and Lucy dumps him for his brother. Fortunately Louisa falls off a wall and in love with Capt Benwick instead.

The unscrupulous Willoughby, on the other hand, displays a clear preference for Marianne, although not technically getting engaged to her, but then he publicly dumps her for a richer woman. She has no father to defend her reputation. He is a cad.

She felt the loss of Willoughby's character yet more heavily than she had felt the loss of his heart.
Sense and Sensibility Marianne Dashwood, Chapter 32.

"When a young man, be he who he will, comes and makes love to a pretty girl, and promises marriage, he has no business to fly off from his word, only because he grows poor, and a richer girl is ready to have him. Why don't he, in such a case, sell his horses, let his house, turn off his servants, and make a thorough reform at once."
Sense and Sensibility Mrs. Jennings, Chapter 30.

Which is all to say that there was a lot more at stake in those days.