What I'd like to see is a companion actually rejecting the PC during or right after attempting to flirt or 'making advances'.
I can imagine something along the lines of (and it's just one example):
Male Andromeda Protagonist: " You know... you look great in this attire. * smiles subtly * " I particularly liked how you dispatched that alien trying to grab you by the legs. You're quite... effective... * winks * "
Female Companion: " * hesitantly replies with a nervous voice tone * Oh, thanks, hehe. * breaks eye contact and nervously looks around the cabin * "
Male AP: " * approaches her slowly * The way you took care of that alien on our last mission wa- "
Female Companion: " * gently stops him from getting closer with both hands on his chest * Look, you're a good guy but I'm already engaged. "
Male AP: " * adopts a surprised expression and comments * You are? It's... someone in the station? "
Female Companion: " Wi- with due respect, sir... I don't believe that it is of you business. Sorry, but I am rather incomfortable right now. * starts to walk away *
Male AP: " * comments as she walks away * I didn't want to intrude in your personal life, but I read everyone's dossier. And yours made me believe that you were single. There's no mention of a partner that would have been part of the crew in this expedition that I would have had to be aware of, and respect. I'm in the unknown here.
Female Companion: " * stops and turns to face and look at him, takes a slower and deeper breath before replying * I chose not to mention it, sir. "
Male AP: " So... you lied. * followed by a moment of silence as both stare in each other's eyes *
Female Companion: " Writing 'confidential' would have sufficed? "
Male AP: " * hesitantly nods slowly * I would say yes, and definitely better than just leave a blank spac- "
Female Companion: " I'm lesbian, need more details? Just don't go around revealing it to the entire crew... sir. "
Male AP: " * jaw slightly drops, speechless, eyes wide opened * Oh... I, well I didn't know that, either. "
Female Companion: " * rolls eyes * Tsk... well now you do, and it's personal. "
Actual player who wanted to bang the chick: " WTF BioWare! WTF!!! "
BioWare: " I made that character, I decide, go cry me a river * /ends internal thinking * " Here at BioWare we believe that artistic integrity is of utmost importance. "
* Manly tears *