Sisko should have found a way to turn down mirror-Jadzia's advances. Sleeping with her under false pretenses raises consent issues. He should simply have refused. There would be exuses that did not threaten his cover... Claim erectile dysfunction, claim an STD, claim to be too tired, simply say no to the sexual advances. If this offends her, so be it.
Regardless, it was wrong for Sisko to sleep with mirror Jadzia under false pretenses. I was very dissappointed in him.