So you praise intelligence and blame animal instincts. But who told you that you are right? ... Wouldn't humans without sex be not quite humans any more? .... This is another reason why it is necessary that science interfere with natural mechanisms
sexual matters are one of many natural instincts, without those insticts, we can't survive, that is why Mother Nature put them in place. Lust is a human concept attempting to moralise sexual matters, refering to those matters as animal instincts is an attempt to debase nature; do we really need to debase and moralise natural functions? It is natural to find more than one person sexually attractive; lust and moral issues only come into effect if we act upon those insticts in a society that frowns upon taking more than one partner.
Explain the one positive that comes from one person that breaks apart the cycle and seed of family for that more than one person 'animal instinct'. it happens in the animal kingdom, but us humans have the 'instinct' of right from wrong.
It is an animal instinct for guys to want to have sex with different people all the time because in "cave man days" that was their job, it was important to get women pregnant so they could have lots of offspring and keep the population up, it was also important for the women to find a man that would not only get her pregnant but would stick around to protect her and the child once it was born.....which brings us to today where its still basically the same concept. except it is no longer socially acceptable for guys to get a bunch of women pregnant, it is still an technically an animal instinct, though not really an acceptable one nowadays