Heh heh, another loaded question with many answers.
First, some background. I'm re-reading the late David Eddings' superb series "The Belgariad." I'm up to book 4 now, in which our intrepid heroes meet a religious zealot. Relg has a load of spiritual baggage heavier than a fully loaded semi-truck. One of his most consuming preoccupations is his battle against sexual sin. To Relg, everything is sinful, and he spends a great deal of his time beating himself up for it. He seems to feel that looking at a woman with impure thoughts is the "worst" he can do to sin against his God.
It's like that for many Christians, isn't it? What is worse in our Father's sight: having inappropriate ideas about a member of the opposite sex, or ...
...lack of charity?
The truth is, we simply don't know. Still, many of us seem to portray the inward idea that sexual error is worse than anything else we can do to grieve the Holy Spirit.
Yes, He wants us to walk in purity. That's a given, and foundational to our faith. But does He not also want us to walk in humility? in peace? in acceptance? in charity? in spiritual industry?
What's worst? Maybe it's time I, for one, got my priorities a little better aligned with His.