"If an action would be against God's will, then I believe nurturing the thought would be against God's will as well."

This is a huge issue in my eyes.

If you have a strong desire for something... and you want to avoid doing it because it would be sinful... is it ok to use fantasy to vent the pressure of the desire?

It seems to me, that if it's also sinful to vent with fantasy then we are caught between a rock and a hard place. Our options are:
1) Vent without fantasy. I struggle to see this as possible (at least for me).
2) Go into heavy abstainance, where we try and avoid focusing on every sexual thought and feeling... sweeping them out of our minds as quickly as possible and distracting ourselves with other thoughts. Sounds noble and pure... but is, I suspect, quite impossible and bound to lead to failure and condemnation whilst doing very little to curb one's desires.
3) Find a way to rid ourselves of the desire. This is the path the elders of my church try to tread... the don't however have any clue as to how it's achieved.
4) Accept that we are sinners saved by grace and that we are weak in this area and while we resist as best we can... expect that giving in from time to time will be a regular and inevitable occurance. The problem with this view is that it's by grace that we overcome sin.... not just cover it. There has to be freedom from sin in Christ. We can't say that any one sin is impossible to deal with... I would think.

I would expand your statement to something like this:
If an action would be against God's will, then I believe nurturing the thought of actually going out and doing it would be against God's will as well.... but using fantasy with a view to venting the emotion and curbing the desire SO THAT we aren't tempted to go out and do the aciton is not sinful.

God have mercy on us all
Blessings
Cat.