There's a certain option at the end of 9th chapter, where you talk with companions at the cantina. Lower version is flirt, which starts romance, and upper version, where you can volunteer somebody to talk with your character. Most people would think that upper version is flirt option with Theron, so they aren't locking their romances.
I didn't lock it on my main, course this choice is very unclear. Will there be another opportunity to lock romance in the future chapters?