Listen, I like a good will they/won't they storyline as much as the next TV lover. Probably more, actually. I've watched many a television show well past the culturally agreed upon drop-off point simply because I wanted to know if two characters with sexual tension would finally give into their urges and get it on. Right now? I'm way too invested in Richard and Monica's potential romance on Silicon Valley, for example.
Unfortunately, sexual tension, romantic chemistry, and cliffhanger couples don't always work once they actually get together. Sometimes it can actually ruin the show, but usually it's just not as interesting to watch two former sparring partners all loved up and happy. Plus, any TV writer will tell you it's easier to create stories out of conflict and tension than hearts and flowers.
The following television couples are all examples of this phenomenon. Here are just a few times the audience wondered "will they or won't they?" but then they did, and oof...they shouldn't have.