If Thornton had just been sitting in the hot tub like a normal person, I do not believe she would have been upset, even with all the girls there. But she arrives on the scene just as he is underwater getting an up-close look at the coeds' hoo-has, then when he comes out, he says "Now that's what I call Marine Biology!" Then when he sees Diane, he says something which is a) gross and b) grossly insulting to her intelligence, namely "Say hello to my nieces!" The combination of his icky behavior and his completely inappropriate excuse are what pissed her off.
As for Philip, she tried to get him to go into the party with her, but he was too much of a stick-in-the-mud. There is no reason for her to pass up some fun just because Philip won't go.
In terms of the dinner where she stood Philip up, she tried to call him to let him know she was going to keep tutoring Thornton over dinner. Evidently she couldn't reach him. Remember this was 1986, before the widespread use of cell phones.

Cause he was taking bath with Bubbles.

What's good for the gander is not good for the goose.

I think she was going there, dancing and feeling good, then got a kick in the gut when she didn't get what she wanted. I probably would've been a bit bummed too, especially when he just jokes about being with his nieces. I think her being upset isn't that surprising.

She shouldn't of been so upset. After all, he was with his nieces.

I think she was just shocked. She was on her way toward falling for Thorntonand leaving Phillip in the dustso she couldnt help but react the way she did. Im not saying Thornton shouldve acted any differentlybecause youre right, they werent datingbut you cant fault a person for feeling what shes feeling. Its not like she held it over his head either. Their reading to each other served as proof of that.

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I thought the same thing. It was awfully hypocritical of her to be upset with Thornton for fooling around with coeds when she was openly dating Professor Frigid at the same time. The biggest reason she liked Thornton was because he was a fun-loving free spirit, which is what she seemed to be upset about.
As much of a pompous ass as Philip was, I kind of felt bad for him in this film. He's justifiably upset that a guy like Thornton is able to skirt admission requirements and skate his way through college using charm and bribery. Then Thornton acts like a know-it-all in Philip's class, undermining his authority as a professor, and to top it all off, Thornton steals his girlfriend right out from under his nose. I can't blame him for being pissed off about that. I certainly would be, in his position.

I am looking in rochelle's direction. I think every person, both male and female have two sides to them. They have a fun, reckless side and a serious, professional side. Rodney can be a good guy and a fun guy. Phillip had almost all stodgy, nose-in-the-air perfectionism. Diane wanted both from the same man.
Enter my contest! I need help for a new signature! Maybe I'll choose yours and you'll win a cash prize!

I don't get what she even saw in Philip in the first place. She was a free spirit and wanted fun and passion, and Philip is as far from that in every possible way.

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Obviously she had feelings for Rodney but hadn't expressed it yet. After all, why else would she go to a young kids' party like that?
She was falling for him and felt a bit betrayed when she saw him in the midst of all those women. She wasn't right to be mad, but then, isn't that rather typical of women?
Also, by how Rodney had been acting and how he was courting her (in a gentlemanly fashion), she thought he was more of a person than that. Seeing him in the hot tub with a bunch of bikini girls made him look like the typical guy who only cares about boobies and buns.
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Diane was so upset at the hot tub scene because she thought that through poetry, she could actually tame the inner child within Thornton. Silly women, always thinking they can change the men in their lives!
Just saw this movie again last night for the umpteenth time and never get tired of it!

Previous to the party she had only been with Thornton in the classroom and maybe the night they came back from dinner. Remember when they were at the Italian restaurant he opened up about how much he loved his first wife. That was a serious discussion, and Diane probably never saw him as a womanizer, rather just being incredibly immature. I think when she walked into that party she expected to see him at least dressed and greeting his guests with a martini in his hand.

Hey!! My fellow Bachelor can!!

lol again - don't feel sorry for Philip: he was a pompous jack-ass.. the actor who played him always specialized in a character of that sort..
Dr Turner is a satirical mock-up of an 80's 'liberated' woman - she got over the fuss at the party, and continued helping Melon - later, we see her tutoring Melon while they cuddle..
canuckteach

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YES! I came here to post this exact thing! There is NO reason for her to do that to Philip! Definitely wasn't cool with the fact that the movie advocated that kind of thing.
And this part of the post is just my signature.

The one time I actually felt bad for Philip.
He had a date scheduled, his date shows up and walks by him and completely ignores him. He asks says in disbelief, "We were suppose to have dinner."
"I just had dinner"
"I don't believe this"
"maybe it's a dream."
And to top it off, she blew him off for his nemisis.
Sure, he had it coming to him, but she's b*tch for doing that.

I totally came on to ask the same thing. What a beeotch. And she was actually dating Philip. So typical of a woman to act that way though. Well, some women anyway.
"I'm boring and my slide show EATS."

I don't get it eithershe goes on about how first men were too macho then they were too soft, he was just being his regular self, a facsimile of a man.

Exactly right Rocky, my friend!
At times like this, I am often reminded of a line from "Camelot" King Arthur says, "Merlin once told me, 'Never be too terribly disturbed if you don't understand what a woman is thinking..they don't do it very often.' But what do you do while they're doing it?"
"I can arrange for you a humiliating death. And, so we're on the same page, it will involve sheep."

Dude, are you actually trying to find logic in a woman's thought process?
"I fulfilled a lot of people's predictions about me. I've become a real scumbag."
Danny Vermin

Meanwhile she's playin' the dorky professor guy.