1. I wonder if Darwin and others considered the possibility that many species have sex for reasons other than reproduction.

In evolutionary science (at least the kind I try and focus on) we try to understand the function of a phenomena (e.g., the adaptation or exaptation). Sexual patterns fundamentally exist as they do in different species because of reproduction (that is the point of sex from a biological perspective - that's why it exists in the first place ). Believe it or not there are hardly any species that have sex that is detached from reproduction (not because it is enjoyable - but because it costs a lot of energy). However, for complicated reasons this is not the case of humans and our closest relatives (i.e., bonobos and chimpanzees) and understanding the reasons why this is the case is important and evolutionary scientists like Eric Johnson do care and study them.

2. I wonder if Eric Michael Johnson, the author, can write an article about sex that carefully defines his value-laden words such as "infidelity" and "cheat."

You can say any word related to sex is "value laden". Evolutionary scientists - and especially anthropologists - do not attempt to inject any value into words like that (even if it can be unavoidable sometimes). And remember he is trying to disprove the hypothesis that women are just coy and monogamous - so he must discuss things like "infidelity". At the same time I don't think Johnson ever insinuates that "infidelity" is in any way wrong. So even if the word is value-laden in our society - he still attempts to remains as objective as is possible.

How can he even use the word "cheat" with all its nonsensical associations when talking about langurs who probably did not have a pre-nup agreement.

First he is referring to what "Hrdy" reported. My supervisor for my Masters studied sexual selection in primates and she would have just used the term "sneak copulation" for a behaviour where the female has sex with a male that does not invest as much time and energy as another male (who also is unaware of the copulation and would be aggressive and potentially violent if witnessing it). So I think this is a fair use of the word given the context and I believe Hrdy's observations (which were that females do "sneak copulations" in many different species (including humans)).

the word "promiscuity" is also loaded with negative connotations.

Perhaps we just differ here but I don't think you can fault someone for using words that society has deemed value laden when he tries to use them objectively (i.e., not insinuating promiscuity is a bad thing).

Final comment: Cadell, I've been meaning to thank you for all your posts, both the ones you write and the ones you link.

Thanks lil! Glad you like them!