Poe/Jess
You're not the rookie, you're a rookie. And you will be until I can convince Finn to learn how to fly. Then he'll be a rookie.
It's not so much that it can go wrong - it can, I've seen guys get shot before missions for wronging somebody they slept with - but more
You saw how many people we walked away with at the end. With Starkiller. That's... nearly every fight. The mission that sent me to General Organa, my wing-man was killed by the First Order and the Rebellion just shrugged. But I'd been close with him, close, and then he was gone and nobody cared.
I'm not saying you should follow my lead here, I already care a lot without sleeping with them, I don't know if I can set myself up for that level of hurt again. But, then, knowing me, I probably will.