I'm having that same problem. I got my character a girlfriend and then started working on getting a boyfriend, all his stats were up and I was going on a date every night for weeks but the prompt never showed up. Finally, I broke up with the girlfriend and next date? He finally asks me to be his partner.
And while all this is happening, I've been stuck in a loop with the Best Friend, I rarely get the option to even talk about sex (despite my character's stats) and when I finally do it's never the scene that resolves their storyline. I guess I'll try breaking up with my character's boyfriend to see if that works, but it's really frustrating if choosing to be poly locks me out of actually being poly.