They probably should have talked. There were a lot of things that they should have done. What actually happened was that as soon as Mary was on the couch, Lucifer followed after, and the kiss that was cut off began again, and he felt it unlikely that it would stop at a kiss.
It was wrong. Lucifer knew something about wrong, and this was probably as high on the list as you got before you started whipping out knives. But it was also right, as if the universe as a whole had just said "Finally, get on with it now."