How do you respond to the claim that lusting after women and watching porn is just part of our human nature and part of the natural evolutionary process from primates?
First of all, the person who asks this question is usually not sincere. Proverbs 26:4 advises us to not answer a fool according to his folly. This is one of the questions which does not, normally, deserve an answer. The proverb is that such a question should not be dignified with an answer.
Now, I assume that you are not one of these "fools" who is asking a question, not to seek truth, but in order to justify his sin. So, for you, I will provide an answer!
God have us many desires with the intention that we use them to his glory. However, almost any of these desires can be used for selfish indulgence. If we do this, we turn something which is pure and good into something which is evil. The murder can say "the devil made me do it." No one is convinced by this argument. God gave us a sexual attraction so that it can be enjoyed in a faithful, covenant, marriage relationship. If we abuse this gift by engaging in pornography, adultery, fornication, masturbation, homosexual relations, sex with animals or any other form of sexual perversion, then this is a sin. The alcoholic can cry that he/she is only doing what is natural. So can the thief, liar, pedophile or any other kind of person engaging in rebellious behavior against God.
Whether God gave us the sexual desire or the desire for food or the desire for accomplishment or any other desire by an evolutionary process or simply by giving us that desire directly, it really does not matter. The question is what is right and what is wrong to do. If the scriptures say that lust is evil, then it is evil. "Scientific" arguments that lust is required evolutionarily for species to succeed have absolutely nothing to do with the question. According to God, is it sinful to engage in pornography? Even the reprobates who ask you this question, in their hearts, know the answer to this question. Like I said, this question, when coming from a person trying to justify their sin, does not deserve an answer. It is not a sincere question if coming from someone who claims to believe in God. I would argue that it is not even a sincere question if coming from a non-believer.