Earlier quoted context omitted.
Sex is not separate from reproduction. If you are a man and have sex with a woman then you're consenting to be a father. Most western countries recognize verbal and other non-written contracts. The clearest example I can see here is the UK's law about weather the genetic father is the legal father. If the child was conceived through sexual intercourse then the genetic father is always the legal father.
Sex has been separated from reproduction, at least in the few species we know of that partake in it for pleasure only. Humans engage in a variety of sexual activities that ostensibly cannot result in a pregnancy. You talk about verbal contracts, how is that different from misrepresenting yourself if you said you were on the pill or you had a vasectomy? The verbal contract of consent can be delivered or revoked during…
That's like saying that eating has been separated from sustenance (or nutrition) because people (just happen to) eat Twinkies for pleasure.
People doing something for reasons that just happen to not to be the primary purpose does remove the purpose of an action.