If you romance both, there will be a point at which they'll make you pick. After that, you're stuck. - MA4585159 (talk)
That's how it works in ME1. In ME3, the situation is a bit different; namely, that you're free to hit on as many characters as the game allows you to until you reach a certain point in their character arcs. Responding favourably to a romance at these "lock-in" points locks you into that character's romance and out of everyone else's; once Shepard picks that special someone, you're stuck with them. Ashley's lock-in point is her sit-down at Apollo's Cafe where she talks about her dad, while Liara's is her meet-up at the Presidium market where she reminisces about her mother. Since Liara's lock-in point becomes available before Ashley's, it's a simple matter of putting the hard word on Liara first. Terminator-HIX
However, that is a much less accident-prone system. - MA4585159 (talk)
Well, ME1's "ninjamancing" was the cause of many a complaint on the BSN, and the comparitively convoluted romance mechanic in ME2 could be exploited to allow numerous possible romances to pan out in one playthrough. ME3's approach is straightforward, but elegant. Even so, I am aware of one person that was stupid enough to lock-in Cortez by complete accident and not realise it until he got to Cronos Station... he was quite shocked to see his character rope another man in his bedsheets xD Terminator-HIX
How the hell does THAT happen? IIRC the choices were "What about me?" and "I prefer the females." They couldn't spell it out any clearer if they tried! RShepard227 (talk)
That was my first thought, too! Apparently, even that wasn't clear enough for this dude. He put this playthrough up on YouTube and everything; that's how I came across it. Terminator-HIX
There is a variation to the lock-in. I had a suspicion about the kind of story template being followed by the developers and began wondering about the consequences of various no-romance paths - partly motivated by precisely the kind of annoying surprise mentioned above. One solution to annoying and pointless dialogues (with a character Shepard is doomed to leave forever anyway) is to adopt as love interest a character who is hardly ever there anyway, like Kelly who barely touches the story. But she is only available from ME2 onwards.
A better idea is to kill off the romance character. Most efficiently done by romancing Ashley in ME1 then having Kaidan as the Virmire Survivor. Puts paid to gigabytes of inappropriate conversations. One still has Ash's image in a black frame on the desk, and very little being hit on by the crew. SDoradus (talk)