From the Mass Effect 2 Wikia [Romance page][1]:

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. [1]: http://masseffect.wikia.com/wiki/Romance

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.