I won't quote anyone specific because I think multiple people in this thread are having a hard time seeing us from their high horse. It's just sex people, yes that's right it's just sex. As long as you are safe about it and you aren't bringing an unneccessary life into the world who gives a flying fuck? But then this is coming from someone who thinks open relationships are fine because sex is totally possible and acceptable without feelings.
I think you're cutting corners here. You're basing your point on an argument that should have been the discussion in the first place. You can't just say "it's not a problem, because sex is just sex". We can deduce that if sex is just sex, then it wouldn't matter. But if it isn't, then it would matter. And it matters to a lot of people, so by definition, sex isn't just sex. Else I could go out and fuck whoever I want, and my girlfriend couldn't get upset, just as she wouldn't get upset if I just spoke to someone. Because it's just a conversation. This is not the way we seem to be wired, based on the fact that most people get hurt when their partners sleep with others, and it's complacent to displace these feelings on such things as "cultural norms" and bullshit like outdated social structures.
So no, it is by no means an axiom that sex is just sex. So you can't argue a point by coming with a false axiom.
It's an interesting discussion, so brushing past the point is even letting go of an opportunity to have a good discussion. The problem by brushing past it is also that a lot of people in a power-imbalanced relationship will use the excuse "sex is just sex" because they have a desire to sleep with others, and giving a sort of "deal with it" to their SO who might not be on the same page. I have no idea what percentage this is of open relationships, but we should all be able to see that it's at least present to some extent.
Other than that, it also seems to me as a sort of overindulgence. No one can say you can't jerk off 5 times a day, no one can say you can't eat 10 000 kcal a day. No one can say you can't work out for 10 hours per day, either. It's not morally wrong, but it's giving into an urge. Most of the people that do give into urges start making excuses for themselves. Fat people say such things that they don't care for living if they can't eat what they want, others say they find it healthy spending 10 hours on NeoGAF every day. Our minds are brilliant at filling in the things we don't want to do, but do because we can't stop giving into our urges. Smokers will say they enjoy it so much they don't care to give it up.
Indulging in sex in the same way is definitely no different. It's the same sort of primal urge as comes with any addiction, be it for gaming, gambling, drinking, eating sugar, doing drugs, smoking. The great porn study talk on TED should give you an insight as to how easy it is to get addicted to sex and porn, and if you do share your life with someone that's in the same position as you, it does not mean it's healthy to say "sex is just sex" and sleep with others. You've put it up there as a physiological need. It isn't. You need food. You need air. You don't need sex. Not giving into urges is not the same as disallowing basic instincts. No one will argue that it's not a high priority to have sex, but your balls won't explode if you don't sleep with someone or jerk off, so you can't say it's a physiological need. It puts it up there with any other form of addiction.
It is good that we identify with certain "urges" or "drives" if we wish to call it that. I take joy in feeling lusty. But showing restraint is also something that's equally important in life. Not too much, not too little.
Is not too much sex sex with different partners? I don't know. I often see girls I think to myself I wouldn't mind banging. Sometimes I can even say "I wouldn't mind banging her". But I do mind. I restrain myself out of respect for my girlfriend, and I do it for my own health, just as I restraint my sugar intake and junk-food intake. I can see many ways open relationships can lead to an addiction. You become reliant on sex with others, because it's your way of spicing things up. I try new stuff in bed with my girl to reach the same goal. One can be addicted to the chase of going out and conquering a new girl, taking her home and fucking her, but it doesn't mean that there's anything wrong in restricting ourselves from doing that. The strange thing with addiction is that it drives us so bad. I've often felt the grasp of sugar. I just can't stop. When I do manage to restrain myself, I can look back and say "wow, that really had a grasp on me" - and we can be there with sex. Going on blindly and saying it's in good health and fun, and "who cares" should
never be without reflection, and a careful insight as to if this is the same thing as a sugar-addict is doing when he picks up yet another candy bar from the shop. "What harm can one more do?"
These things are tangled with cultural norms, physiological reactions and craving for sexual conquest and orgasms, but implying that evolution has not bound sex and feelings
closely together is an ignorant one, and one that sounds to come from a stem of wanting sex not to be tied to feelings out of a way of upholding a way of giving into urges. Are we making excuses for ourselves, or are we living life to its fullest?