Romance Mechanics
Romances in Mass Effect 3 are much more straight-forward than in the previous titles. Shepard is free to answer favorably to one or more love interests at a time, up until a "locked-in" dialog choice is presented at a specific location. Should Shepard choose to answer favorably at that time, the dialog options for all other love interests will change - with the exception of Kelly Chambers. The game doesn't perform a check to see if the player is "locked-in" with Kelly for the other love interests. If the player "locks-in" with Kelly first, the player may choose to pursue a simultaneous romantic relationship with another love interest with both love interests unaware of the player's other romantic relationship. In other words, love-triangle confrontations do not occur, but the player can be a two-timer.
Example "locked-in" locations are:
Steve Cortez: Kissing him when dancing at Purgatory after convincing him to take some shore leave on the Citadel.