I see what you're saying in principal. But it's not the same. Because the difference between just plowing a girl and leaving it up to the Fertilization Gods and using birth control are HUGE.
The difference between your chances of getting injured between playing with the best OL and the worst OL is SMALL.
As I said, the chances of any QB getting hurt is like less than 3%. Just going to town on a girl and the chance you knock her up is like 80%. The drop off from playing behind the best OL only really drops it to <1%, while using almost any forms of contraception is going to drop your chances of getting your female partner pregnant to under 30%. Just trying to not get a girl pregnant, alone just pretty much halves your chances of her getting pregnant.
According to Pro Football Focus, Peyton Manning has seen the least amount of pressure this year, thus is the best protected QB in the NFL. Yet, 1 hit still hurt him:http://www.cbssports.com/nfl/eye-on-foo ... ned-ankles
Again, the random part is which hit will do it. A QB might get hit 110 times and not get injured, but then on the 111th hit he goes down (see Ryan last year). Or a QB could get injured on his 3rd hit of the year (see Tony Romo this year). In the case of Manning, he's seen half the pressure that Ryan has seen this year according to PFF (67 vs. 112), yet Manning got hurt thanks to one of those pressures/hits, while Ryan has not.
You're right, the less he gets hit, the less chance he gets injured because he's exposed to less hits. I don't really see the pregnancy analogy applying here. Considering we've evolved to where our bodies try to maximize the chances you get pregnant, and we haven't evolved such that in football with QB hits. IN fact, the rules have done the opposite to significantly lower the chances of a QB getting hurt.
As I've said before, there were about 20 injury-making hits on QBs last year in 256 games. You should be able to sleep comfortably knowing that.