On the question of fertility - sperm count - I feel that a thin bull AND a fat bull, will have lower fertility/libido, than an average BCS bull.
Just like anything - too much or too little is not good.
I keep referring to our bull test (mainly because I am currently working on all the final data as it comes in). Anyway, we set up the test so that the bulls have 6 weeks of "cool down" ration prior to the sale. They finished the test on 3-10 and the sale will be 4-29. By sale time, they are on a heavy roughage diet. And, as I said before, they are "discounted" for being overly fat (not a lot - 5% toward backfat index & 5% towards intramuscular fat) (30% ADG, 30% WPDA & 30% adj 365 day ribeye area per 100# body wt).