Audrey's love at first sight for Jack is totally romantic, and appropriate for a teenager. When she met Cooper it was 'OMG a handsome FBI agent.' Enter tall, dark handsome (wealthy, charming) stranger ('OMG, hottt!)-> heartbreak is cured. Happens in real life all the time. Audrey was pretty spontaneous about wanting to fall in love, lose her virginity, and run away with whoever from day 1.
Cooper is physically attracted to Audrey (who wouldn't be) and cares for her like she's this fragile, china doll. And she has a wicked, schoolgirl crush on him (duh) but he doesn't meet her passion halfway.
It would have been an interesting affair, but she was way too immature for him. There's more of a symbiotic balance with the Coop/Annie, Audrey/Jack relationships.
I actually disagree 100% with this. I can see it on a superficial level -roll identities and setting up the dynamic. But the twist of Twin Peaks was peeling the layers away and revealing no one is what they appear to be on the surface. If anything the pilot sets up Audrey not as a romantic but a calculating schemer, and yes, Cooper might be attractive to her, but he in essence captures her and makes her look more inward. It's still playing around with the school girl crush tease until the rug is pulled out from us and Audrey is revealed to be crying while watching Leland dance -the surprise there being the town bad girl is actually the most innocent and pure.
The refusing to sleep with her is for tension and deepening the bond- the flirting arc tease is now replaced and deeper and darker and more complicated (again, the essense of what Twin Peaks was all about) -and clearly they wouldn't set this up as a crush by having Lynch shoot Fenn praying to Cooper, and intercut him in his dreams from bed. What does it do narratively to have it be just a silly crush on her end? Her longing heart was the essence to the character. And the whole fun of it was Cooper couldn't stop thinking of her -"Can't think of clues, but on the content of her smile," and his cloudiness also puts her at risk and draws on his mistake from Caroline. (2.06) Yes, it would have been messy and complicated -but that would have been the fun.
And I don't see the immature angle at all in terms of Audrey -the writers, particularly Peyton loved writing for her because she was supposed to be wise beyond her years, just impetuous and impulsive ("impatient, impatient for her life to begin and the world to catch up with her dreams"; "Laura always got her way ...just like you.") On paper, I'm sure they didn't plan on Cooper bedding a teenager -most likely the chemistry leapped out between the two actors -and the public latched on -Fenn and MacLachlan were the two favorites for most of the run.
The "symbiotic balance" is nice in real life but not on TV -and Annie and Wheeler only balanced them because A) Audrey and Cooper were the stars of the show with the most press and therefore needed love interest plots, and B) there wasn't much time left so they needed to expediate the love interests and bring both pairs together quickly with their respective new mate. So each new love interest would be as easily drawn to and unified with Audrey and Cooper. The result was no sense of satisfaction because it felt like it wasn't earned but merely plot contrivance. Watching it in real time with an episode a week, there was this incredible yearning from many of the fans -anxious to see the Cooper/Audrey scene of the week and what would happen. And I would say since Peaks is about light and dark and the two flipflopping, Cooper and Audrey were more in keeping with the theme (The Good Cop, The Bad Girl and peeling those layers away and delving into their pysches).
And since the continued Cooper/Audrey saga (no matter what is was) -was only halted because of MacLachlan, it is obvious proof that is was part of the intended story as a whole. Isn't this the same thing if Ray Wise decided, "Oh no, I disagree. Leland would never have killed Laura and I don't want it to go in that direction, so come up with a different solution. Sorry."
Something most likely was going on behind the scenes -and it probably had to do with Fenn's temperment and the amount of press she was getting, the award nominations etc, and becoming the fan favorite. But MacLachlan's excuse about it compromising Cooper's character seems out of the blue in comparision of how he played the early Cooper/Audrey scenes -and hell the original draft of 1.06 had them at breakfast together after finding her naked in her bed, so the writing was on the wall back then that Cooper would be tormented and conficted with his feelings for her.
Bottom line, the two were always supposed to be connected narratively. And to me, when this stopped, I stopped caring.