Mass Effect 3 will have no new romances. Instead, it'll use save files to include the romances from Mass Effect and Mass Effect 2, and if you have two different romantic interests from each game, things could get hairy.
In Mass Effect 2, players were briefly in contact with Kaiden and Ashley from the first game, while Liara was a subplot - and later the subject of the Lair of the Shadow Broker DLC where players could re-ignite the romance. If the player remained faithful to their lovers in ME2, they lovingly looked at their photo before the final mission. If they went with a new love interest, that photo was placed face down on Shepard's desk.
In Mass Effect 3, if you romanced different characters in Mass Effect 1 and 2, that romance has to be resolved, which means you might have to, say, choose between Jack and Ashley, or Jacob and Kaiden. Things could get... messy. Of course, if you remained faithful to one character, that situation will be played out.
And if you romanced no one between the two games... what's the matter with you?