I rated that post E because I think visible dicks should be an automatic E. I recently went through penis rating:q and re-rated a bunch of blatantly exposed cocks from Q to E.
This particular post is debatable, but in general, I think if something is borderline you should go with the higher rating. There's no harm in rating something a little too high. There is harm in rating everything too low.
This ties in to topic #18572, but basically the rating system has gotten extremely diluted over time and I find this to be very frustrating. It used to be that visible genitals were an automatic E. The help text on the uploads page used to say that E was "hardcore porn, visible genitals", Q was "Nudity, anything erotic", and S was everything else. But over time the standards have lowered to the point where now even blatantly exposed genitals aren't E. I see things like throbbing futa cocks or closeups of a person's gaping asshole being rated Q for a month and it drives me nuts.
I'm sure some will say that sure, blatantly exposed genitals should be E, but "simple" nudity should be Q. The problem with this is that if you don't have simple, clearcut rules like "visible penis = automatic E", then everyone will say that their posts are tasteful and artistic and definitely not porn, and the system will gradually decline as everyone pushes the rating standards lower and lower.