Phasma and Kylo get starship-wrecked on a dangerous planet and, despite themselves, grow attracted/attached/etc. to each other. They decide to bang once to get it out of their systems, then find the attraction unfortunately persists even after they're rescued.

I'd prefer an emphasis on Kylo and Phasma actually being annoyed by their inconvenient attraction rather than on them developing genuine romantic feelings for each other (though I'm not opposed to feelings).

+Hux catches them having a quickie in a storage room and is horrified/angered/disgusted/intrigued/all of the above.
++Phasma and Kylo's idea of "dirty talk" is cutting insults.
+++Somehow, despite the fact they don't really like each other that much, the relationship works.