Or is it?
For some reason, happiness does not appear to be coming with the coupliness. Blame the Rule of Drama. Traits they formerly laughed off about each other are suddenly causing long bickering sessions that can result in one party storming off while the other stands there and cries; stuff they used to enjoy together has become boring and immature, since they're a couple now and should do more couple-oriented things (this view is often posited by female characters, and usually leads to more bickering).
To sum up: they got together, now they fight. Sex just changes everything. The polar opposite of Sex Equals Love.
This can be really jarring for an audience that's used to this pair being friendly and supportive of one another. More often than not, these characters have never fought like this with each other before (and sometimes, they've never fought with anyone like this before). If this happens with a Fan-Preferred Couple, it can be read as a Take That! against the 'shippers.
Can be Truth in Television if Platonic Life-Partners actually do hook up and things don't exactly work out. It's the reason some people say I Don't Want to Ruin Our Friendship, and other couples realise they really are Better as Friends.
This is a specific subtrope of the Reset Button.
- A major part of When Harry Met Sally.... Though they weren't fighting over petty things as much as rehashing things to figure out how they work as a couple. Still an example but not as petty as other versions.
- In the fourth The Sisterhood of the Traveling Pants, this happens between Tibby and Brian. They had become a couple at the end of the previous summer, but didn't have sex until the beginning of the next summer. After a mishap with a condom, Tibby believes she may be pregnant and promptly freaks out. They don't fight so much as she refuses to talk to him.
- Happened before Cyber Joly Drim starts, but the protagonist's husband remains blissfully oblivious.
- On Scrubs, when J.D and Elliot hooked up the first time, the entire next episode (which was unusually long when originally aired, no less) was devoted to having them essentially tear each other apart until they called it quits.
- Ross and Rachel of Friends: A. Ross and Rachel kiss. Everything'll be great now, right? Ross writes a list of Rachel and Julie, pros and cons. This infuriates Rachel, and she dumps him. B. Later on, they get together for real. The relationship lasts the better part of a year, but Ross gets jealous of Rachel's new job and her coworker Mark. Some misunderstandings and a sleepover with the hot copier girl, and they break up. C. Later on, Rachel says that she'll be with Ross again if he accepts full responsibility. He agrees not knowing what he was agreeing to. They break up again. D. They get drunk in Las Vegas and get married, then divorced. E. Rachel gets pregnant. Cue fighting about whether to get married or not. I've probably skipped a few steps in here, but you get the idea. The creators dangle the carrot in front of us and yank it away over and over again, knowing we'll fall for it every single time.
- And possibly because of the sheer frustration of Ross and Rachel's relationship when Monica and Chandler had sex, none of this happened. When they argued it was mostly about legitimate couples stuff and they resolved it more often than not by listening to each other and being mindful of the fact that they were different people who would occasionally differ on things. This was partly because they were extremely close prior hook-up but unlike Ross and Rachel weren't hiding hopes for 'more' and starry-eyed. Instead Monica and Chandler were well-aware of each other's faults but still best friends who openly admitted they loved each other. You sensed for them the step from platonic to romantic love was a natural one, as they had a stable basis for their relationship.
- In Buffy the Vampire Slayer, sleeping with Buffy triggers the loophole in Angel's curse and robs him of his soul, turning him from The Atoner into a monster. As if the subtext wasn't obvious enough, Buffy uses the mundane version of this trope to explain away to her mother how she could have slept with someone now so evidently psychotic.
- Betty and Henry on Ugly Betty, resulting in Shipping Bed Death.
- In Star Trek: The Next Generation, Picard is sent back in time with a chance to alter a critical point in his life. Along the way, he winds up sleeping with someone who he'd previously only been friends with and finds he wound up destroying their relationship as a result.
- In Doctor Who's "The Lodger," after the Doctor gets Craig and Sophie to admit their feelings for one another, they decide to "totally ruin their friendship." In the sequel episode the following year, they have a little boy named Alfie, or as he likes to call himself "Stormageddon, Dark Lord of All."
- In Mass Effect 2, Jack will offer you casual sex early on, but if you take her up on the offer, she will subsequently refuse to talk to you. If you choose to romance her, no sex is involved at any point until Mass Effect 3.
- A Getting Crap Past the Radar PG version happens in Tales of Xillia 2. The spirit Muzet talks about her experiences with direct tethering, which involves a human filing a spirit with their mana. When the usually stoic Gaius realises the implications of what he's done with her, he is absolutely mortified and starts rambling to himself about how he has to take responsibility.
- After getting deflowered in the end of Leisure Suit Larry 7, Shamara undergoes a complete personality change at the beginning of LSL 8.