What can I use to lubricate CAT6 cables while installing them?
When someone says soap you have to take in consideration their experience. They may very well mean soap, as it is slippery and would probably work. If, however, they're from the electrical/low voltage field they more likely mean Yellow-77 - or a similar variation available today. A gel-ish type is what you'll likely find at a big-box store. Personally, I prefer the yellow stuff, but I'm old-school like that.

I doubt sex lube is what you want. It's probably expensive and not really the consistency you're looking for. But it might work well enough if you've got an abundance laying around.

Ideal : Yellow77

When applying a pulling lube don't fall into the trap of thinking the entire wire must be saturated with the stuff. You shouldn't need much more than an initial amount on the head. It will work to coat the path for the following wire. This is why using the proper lube is beneficial. It's consistency works for you.