God, since the beginning of time, intended for his creation to be fruitful. Whether we are referring to Adam and Eve, or Noah's sons and their wives. God has always desired that two believers unite in marriage and produce a godly seed, according to the bible.
Without sexual attraction, procreation/reproduction would not only lose its allure, but being fruitful and multiplying may very well become unpopular. God did not create us as asexual beings, but rather, beings who are able to feel, sense and enjoy pleasure.
The times of intimacy between a husband and wife are very private cherishable moments reserved for them alone. Sexual desire is normal, but unhealthy, immoral or ungodly lust, is a desire to possess something through imagination or physical involvement. The bible states clearly that our bodies, are the temple of the Holy Spirit. It also says, that the body was not made for sexual immorality, but for the Lord. So, its not simply the natural, God given desire to have sex, that should cause one to feel that they've sinned, but what occurs in the mind and with our bodies, that we should be watchful for.