I don't think your ME2 love interest would leave you just for finding out that you were with someone before, especially since all of your love interests are aware about the whole 'two years being dead' thing.
They'd probably leave you if in ME3 you went
back to your original love interest. [smilie]../../../images/forum/emoticons/lol.png[/smilie] But you already mentioned you didn't want to do that so I don't think you need to worry.
I don't think cheating is a good word for it anyway. You
died and your LI started to move on in your absence. Plus they (excluding Liara) turned their back on you when you
really needed their help. It only could have been more of a betrayal if they had tried to arrest you.
Unless people did this:
Also don't you think the repercussions should be different if you slept
with Liara in LotSB after you romance someone in ME2? That, to me ,is
cheating whereas going your separate ways after horizon and then moving
on is not. I just hope that gets acknowledged.
^ That is definitely cheating and deserves some kind of super dramatic backlash.
Possible repercussion that I think about?Fighting (either between you and your original LI, or between your old and new LI)
Not getting cooperation from your original LI
Your new LI asking you questions about it and/or doubting your relationship because of it.