Why did Maria decide to marry quickly?

If you've read Jane Austen's works, you'll understand that country girls (as Maria and Julia were) took every opportunity to go to town for the society.

The Bertram sisters have had very little society at Mansfield Park because their father preferred to live a more quiet lifestyle. He had very strong opinions about what was moral and correct (too bad he didn't try harder to instill these things in his daughters!). The Bertram sisters had long been wanting their father to give a ball at Mansfield, but he never does, that is, he doesn't until after Maria marries Rushworth and Julia goes away with them.

The Bertrams were wealthy and stylish, but nobody paid much attention the the girls. They were not much thought of as more than pets or posessions.

Mrs. Norris, their aunt, was very indulgent of the Bertram daughters. She showered them with praise and puffed them up. But unfortunately, no one really paid attention to how they were turning out.

Maria and Julia were very close, and of course Julia's well being would have been considered above Fanny's.

Maria and Julia had recently been at odds with each other over Henry Crawford. It caused a huge rift between them, with Julia finally deciding that she didn't care at all about Henry. This is why she started flirting with Yates. The rift between the sisters ends when Crawford leaves town after Sir Thomas puts an end to the theatricals. The tiff is mended further when Julia joins Maria on her honeymoon. As to bringing Fanny along, there is no way that would have happened. Fanny was not considered as really part of the family. She wasn;t a Bertram, so she wasn't entitled to be treated like one.

Maria's marrying someone like Rushworth would put them into acquaintance with some very wealthy and respectable bachelors. It would have been a good opportunity for Julia to find a partner, and also make new acquaintances. Even Maria would have been keen to get Julia away from Tom's friend, who was trying to pay his attentions to her.

Julia contined her "friendship" with Mr. Yates in London, while she was staying at Maria's house on Wimpole Street. I have not had the impression at all that Maria wished to encourage Julia in another direction.

Maria was eager to marry Rushworth because she was being tempted by Crawford. She knew that he could not provide what she wanted, and she was very materialistic.

Sorry, but this is simply not correct. Maria was IN LOVE with Henry. She thought he wanted to marry her. When her father returned from Antigua, she was everyday expecting Henry to make her an offer, to speak to her father. but he didn't, and it wasn't long before he left the county. Once that happened, Maria had to accept that Crowford had only been toying with her affections; he'd never been interested in her at all. Maria was determined to show Henry that she'd only been playing along with him. She was also anxious to have some independece. She had gotten used to doing whatever she liked while her father was away all those months in Antigua. Since his return, she'd lost that independence. So she determined that she could take care of both needs (Showing Henry that she didn't care for him, and gaining some freedom) by marrying Rushworth right away. But Maria truly had loved Henry Crawford, and if he had wanted her, she would have gladly married him despite his not having nearly so much wealth as Rushworth.

I do not think it was ever her intention to have him as a lover, but upon returning to his company, her weak and selfish mind was easily given over to her desire.

I agree with you about the first part. Maria was still angry and hurt about Henry. Their first meeting after her marriage takes place in London, and after Henry has been attempting to win Fanny. Maria is cold to Henry; but she's even more hurt to learn that he's in love with Fanny. When Henry later makes overtures toward her, Maria is quick to fall into a flirtation because she's still in love with Henry, and because her marriage to the foolish Rushworth is so unsatisfying. But I don't think Henry is really interested in Maria. At that time, he's still hoping to marry Fanny. IMO, Henry thinks he'll have some fun with Maria but then go back to wooing Fanny. He still thinks he will eventually convince Fanny to marry him. However, through their own carelessness they are discovered. And since Mrs. Rushworth and Maria never really got along, it becomes very public right away. Once that happens, Henry realizes that he'll never win Fanny so he runs away with Maria. It doesn't last long, though, and he never does marry Maria.