While it will take effort, it will always be possible to reunite with your ME1 Romance. In that sense, 'cheating' does not limit your options in any way. However you will need to overcome an additional obstacle to your relationship.
You can also enter a relationship with either of the ME1 survivors in your squad regardless of whether you had done so previously.
Where your romance choice in ME2 does limit you is in the carryover options. Specifically, you cannot initiate a romance with any of the ME2 romantic carryovers. Unless you initiated it in ME2 (in which case it will carry over), you will be unable to enter a relationship with
Tali, Garrus, or Miranda.