You could never accidentally romance Zevran because he asks the question quite explicitly not long after you recruit him whether or not you are interested in that sort of relationship. If you are already hooked on another character, then you simply say "no" and he accepts it. I did this with my female mage in love with Alistair and Zevran was thereafter a really good friend without any of the innuendo you get if you don't set him straight at the beginning. He even helpfully gave Alistair some advice on how to improve his performance. My girl was touched at his concern (well I was grinning). On the other hand, if you leave it open as a possibility, then he will suddenly offer you a massage.
I actually managed to get a state with Lelianna where I chose her in preference over Morrigan but then swapped to Zevran - my noble Warden was working through the sex manual he'd read at the beginning of the game. Somehow, I think because she'd been hardened, when I told her I was opting for him she didn't take it that badly and I still seemed to get conversations that suggested she was still interested if I changed my mind. I had to be a bit careful which dialogue options I chose to avoid another confrontation but I must say, if it was a bug, that was one occasion when it didn't seem odd - I just couldn't blame the girl for trying, my noble Warden was seriously sexy. So far as I could tell, at the end of the game I was married to Anora, had Zevran as my lover and Lelianna hovering hopefully in the wings.
You would think that DA2 was better because of the heart icon but actually I found it made it worse. The first icon always appeared almost immediately after you recruit the person and because none of the romances had progressed much by then, I tended to feel obliged to click on it because I couldn't be sure if I failed to do so, whether that would prevent me ever being able to start it up later. I always felt it most inappropriate that immediately after Fenris telling me his former master wanted to flay him alive, I was being asked to hit on him. Ditto talking to Anders after he was grieving for his friend (or as you may discover if you are male, lover) and telling you about Justice. Initially I didn't bother with the heart icon with my female Hawke but next time through I clicked it to compliment her on her eyes in Act 1 and then thought nothing of it until Act 3 when she suddenly turned up on my doorstep apparently thinking we were in a relationship. Boy that was weird, particularly as by then my male Hawke had realised he was gay and had just reconciled with Fenris, so you'd think that would have shut off the romance flags with anyone else. Apparently it was very difficult to keep in a relationship with Fenris (you particularly had to watch what you did with Isabella) but very easy to get out of it.
In ME3 I had a very strange time with Kaiden. All I did was visit him in hospital and give him a bottle of booze, like any good friend might do, and then later in the game he says he has feelings for my male Shepard, who was currently dating Tali. Cue awkward conversation putting him in the picture because I really felt bad about giving him the wrong impression even though I was blissfully unaware I had done so.
So if all these are examples of Nijamances, then I could really do without them. I hope we go back to having optional romance dialogue that you can click on at any time in the game if you wish to do so or the companion initiating a dialogue asking if you are interested but after you have got to know them a bit first (like with Morrigan and her "I'm cold"). Even giving you a flower, so you realise they are interested but can let them down gently. Not click on heart icon when you've only just met.