The question you ask is very interesting to me. "Would it help Christians in general to condemn cohabitation as much as we condemn homosexuality?" Do you mean, would it help our cause of being faithful witnesses to God's goodness and mercy? On the face of it, it would be easier to turn a blind eye to all that goes on around us that is wrong or evil; but then we could be charged with dereliction of duty towards God for ignoring the 'plight' of those around us who need to hear the Gospel. It is right that we should be concerned or vexed as Lot was by the unrighteousness around us, 2 Pet 2,7 "And delivered just Lot, vexed with the filthy conversation of the wicked". Sin is sin and everyone is guilty of it but the question for me is how best to convey the truth of this to individuals around me and for the Church as a whole to best bear witness of the fact to those in the nations in which we live.

They are equally as bad as lifestyles but the idea that condemning those choices might help Christians is strange. We should want to show mercy to those who are guilty of these things. Our attitude towards those on the outside of the Church should be one of love and where and when appropriate we might do well to point out that living in those ways is not living God's way. But that is only part of the Gospel as I understand it. Some people need to be told the bad news that they are sinners in need of salvation before they will begin to entertain the idea of turning to Christ and serving Him instead of themselves.
It would certainly let the world see we were being consistent if we were equally vociferous about things in the Bible that God disapproves of.
Another problem I've encountered with highlighting particular sins is that some individuals think, if they put that one thing in their life that's wrong, right, then they will be ok, which is ridiculous. We need to find ways of impressing on folks the simple truth that no matter how 'good' you are you can't please God without Jesus.