The choices between Mass Effect, Mass Effect 2, and Mass Effect 3 carry over, and this includes romances. Some players might be wondering what if they can start a new relationship after romancing a character from the first game, or if it will have consequences further down the line. Many characters are handled on a case-by-case basis, such as Miranda Lawson, whose consequences can be catastrophic.
Liara T'Soni, Ashley Williams, and Kaidan Alenko are the characters introduced in Mass Effect 1 that are romanceable from the get-go. Both male and female Commander Shepard have two options each: either Ashley or Kaidan, depending on player gender, and Liara. However, during Mass Effect 2, they'll be unavailable for various reasons. Kaidan and Ashley will be upset with Shepard for joining Cerberus while Liara is busy dealing with Shadow Broker business. In all instances, Shepard will retain a photo of their love interest in their quarters, despite having essentially broken up.
In Mass Effect 2, Shepard will have a chance at romancing characters that weren't available in the first game, such as Garrus and Tali, as well as some new faces like Miranda Lawson, Jack, and Jacob Taylor. At the end of the day, Shepard can romance any of the folks aboard the Normandy in Mass Effect 3 and still have a chance at rekindling their relationship with Kaidan, Ashley, or Liara later on. Come Mass Effect 3, they'll all be understanding, so this is a great time to get to know someone new, though there are some small caveats.
The most noticeable "consequence" if Commander Shepard romances someone new is that the face of their old love interest will be turned face down. If Shepard decides to break up with their new partner, the picture will turn back up. Additionally, in regards to Liara, if Shepard chooses to stay faithful to Liara throughout Mass Effect 2, they'll look at Liara's picture as the Normandy goes through the Omega 4 Relay.
However, Mass Effect: Legendary Edition contains the Lair of the Shadow Broker DLC, which prominently features Liara. During this DLC, if Commander Shepard romanced a new love interest, even if it was just for a one-night stand, Liara will state that she's aware of it. This provides Shepard with the opportunity to either break up with Liara or reaffirm their relationship.
Interestingly, choosing to reaffirm their relationship still does not limit Shepard from initiating a romantic scene with another partner while passing through the Omega 4 Relay. This seems to have no effect on Liara later on, and Shepard can continue onto Mass Effect 3 to romance her, however, Shepard will be forced to choose between their love interests during the third game. Breaking up with certain characters in Mass Effect 3 can significantly impact their stories for the worse.
Mass Effect: Legendary Edition is out now for PC, PS4, and Xbox One.