from what i know you can flirt with anyone up until the meridian mission once that mission is finished you get a romance scene of some kind at that point you need to pick one and stick with them or it could lead to another ashley/liara fight.
i was reading through some of the comments someone said you can't fool around with peebee if you want to be with her in the end. that is not true at all i already tried this and it didn't effect the relationship at all. nor does it effect any other relationships with other people. peebee isn't jack so don't confuse the 2.
also from what i seen most of the romance comes to an fork after finishing their loyalty missions. if peebee asks you to be with her using her bot your answer will lock you in or out of a relationship with her. same with cora on eos. once those special moments come you need to make a choice. or risk losing all your choices.
so the 2 things to look for are if you complete their loyalty mission and if you have done the meridian mission. those 2 events are what set off the romance triggers that lead to sin city. lol