"Raincoaters" is an old term referring to the good old days of porno theaters. Guys would attend the theatres in long raincoats that would cover up any obvious bulges, and, if they wanted to stroke during the movie, the raincoats would cover up anything they were doing.
So, based on this, I'd say that the "raincoat factor" refers either to how strokeworthy the porn is, or how much it makes one want to stroke.
Astronight may have another definition in mind, but this is how I interpret it.


