Both you and AFT have raised great discussion points.  Christians began to think of sex as dirty when the teachings of Gnosticism [one variety] squeezed in. Gnostics believed that spiritual realities were of more value than earthly realities.  They mistrusted the human body, marriage and all material creation.  Some of them were very ascetical, remaining celibate for reasons of "purity" and adhering to strict dietary rules so as not to cloud their spiritual perception.

Gnosticism [actually a branch called docetism = 'to appear']was condemned as a heresy when it taught that God had not really become a human in the person of Jesus.  These Gnostics believed that Jesus was either a highly enlightened man or an angel who only APPEARED to be human.  

Through the work of St. Irenaeus and others, Docetism began to diminish in importance by 200 AD.  BUT, the purity part, for priests, came back with Pope Damasus [366-384] and continued until celibacy for clergy was mandated in 1139.