Love interest.
Well, they'd been saying "cheating will have repercussions" since they announced save file transfer, which was before the release of ME2, sooooo...
I mean, if you really wanted to romance somebody in the second game without Bad Stuff Happening, was it that hard to create a blank-slate Shep in ME1 (or steal a save file of one off the internet)? It's pretty clear that none of the ME1 LIs thought that it was "over" between the two of you in ME2 long before you had the opportunity to start romancing ME2 LIs.