I have to agree with Donk. He stated the situation very well. If you are heart healthy and have great circulation of blood your penis can become quite erect and even rock hard. That stated, while I do not measure out as strong as a lot of members here I can state that I have to be very, very well stimulated to achieve a boner like the boners of my youth. It is true that as one achieves maturity, the number of boners decreases. Additionally, the real rock hard ones a neat and nice; however, on average I must state that that only happens about once a week. I enjoy sex and sexual activity as much as anyone, but with my pre-cum and loose foreskin I can achieve an entry quite well, even though I am not rock hard every time I engage in sexual intercourse.
We must all remember that the most vital sex organ is the human brain.