You understood what I was trying to say. In case of a cross-generation sexual relationship the child would be unavoidably be hurt because of the current society's influence. I have thought that to be the easiest reason not to have such relationships to prove.
It would be very self-centered and would show a lack of love toward the child if the adult would, knowing the above, begin a sexual relationship with a child.

About the "..we should enjoy sex (...) and the good things of life" part, I believe that following God's will is one of the most rewarding things to do in life. Working for God's glory, perfecting oneself to follow his commands... these are wonderful things to do. They help to strenghten the bond with Him and brings peace to one's spirit (it is an incredible feeling just to know what to do to please God, and then do it). I honestly think I am a happier man than I was before I started the above treatment (and it was what helped me come so much closer to God). Obeying God's will helps get closer to Him, and his presence in one's life is extraordinarily satisfying. And eternal life is a nice bonus ;)

The most difficult part to explain is that children are innocent. I can only rely on my own experience and biographies of other people. What I CAN say for sure, is that young people (let's say younger than 15) don't have experience and that leads them to making mistakes.
For example, at one point in my life, I was a hypocrite without even realizing it. I will spare everyone the details. Long story short, I was using a verse that forbids masturbation to cleanse myself from other sins while not considering masturbation to be a sin.
I think that even though children CAN do many of the things that adults can, they cannot be expected not to make decisions they won't regret in the future. Therefore, one should not make something a child would VERY VERY VERY much regret an option for it. I believe that having a sexual relationship with an adult revealed to the public can turn someone's life 180 degrees in the wrong direction. We cannot expect a child to be responsible enough(because there is a good possibility that it's not thinking rationally, it's not considering all the factors (like me in the above example), or it doesn't understand the consequences) to make such an important decision about itself as entering a relationship with an adult.