How can I control the Romances in Mass Effect 2?

Question

In Mass Effect I tried to get as far as I could in dialogues with characters, even though I was trying to remain neutral as to not get involved with anyone so I could remain loyal to Tali in Mass Effect 2, despite my best efforts to remain neutral but view as much dialogue Ashley came up but I had to blow her off.

Now playing Mass Effect 2 and I'm beginning to worry that my interactions with Kelly may be setting on the path to romance her while when I get Jack I want to be friendly with her however my target is Tali.

I am wondering, in Mass Effect 2 is there a way to control the relationships without having to just flat out ignore characters (so I still get the chance to explore their dialogue), like a particular conversation response that'll lock me with them or something? I'm on the Paragon path.

Answer 1

From the Mass Effect 2 Wikia Romance page:

For a male Shepard, a love interest is "locked in" once you express interest at the following dialogue points:

• Jack: "I want to talk about us." At the point where she says "I'm not interested in some dumbshit love affair. It never goes right." To prevent locking-in, say "Never mind".

• Tali: "I want to talk about you." At the point where she beckons you toward the drive core and says "I've been thinking about the last time we talked." To prevent locking-in, say "So you're not
  interested?" followed by "I need to think about this."

• Miranda: "Can we talk?" At the point where she says "I don't know what this is, if it's just stress, or blowing off steam, or..." you
  cannot opt out; every option results in either becoming locked-in, or the relationship being broken off permanently. It is recommended that the player save before talking to Miranda.

At this point you know that Shepard has successfully 'locked in' that romance. Shepard can still express an interest in other crew members. However, the new love interest will 'challenge' Shepard's love-triangle and force Shepard to dump the 'locked-in' option in order to be with them.

I've only tried to romance Miranda (and apparently after trying that I was locked-in no matter what), but I did also get the Jack dialog.

For more information (about Kelly and Samara/Morinth), you should check out the romance page for yourself. There's a bit about loyalties on there as well.