That leaves a person who contends that men should aim for fidelity two options--show that cheating is not actually adaptive (i.e. challenge the scientist's empirical claim) or decide that what is good for us/right for us is not governed by what is evolutionarily advantageous.
Might one also contest the evolutionary biology approach by pointing out that evolution occurs over such a large span of time as to be meaningless for practical human ethics?
It seems to me that Little Rocker and Godfrey are both onto something. While we are the products of evolution, we don't make choices and rejections based on evolutionary considerations in the here and now in this one life that we have. In this life, infidelity can (and likely will) lead to pain, both in an individual relationship and rippling through societal relationships. If someone is unfaithful to another in a relationship, that unfaithful person can be seen by others as being untrustworthy; hence leading to more pain for the unfaithful partner. There is a chance that there may also be actual physical pain involved should the other party in the relationship exact "revenge" on the unfaithful party. And if not revenge right away, there may be revenge later. It may be easy to commit infidelity undetected, but impossible to be sure that you have escaped detection... to paraphrase a Vatican Saying.
PS...
Quote from Little Rocker‘Men are naturally prone to infidelity because it’s not evolutionarily advantageous for them to be monogamous....
This got me thinking of animals that do pair bond as part of their natural behaviors. And I realize one can't necessarily extrapolate among all these species nor does the concept of "infidelity" translate across species, but a purely "it's natural" (above and beyond the universal feelings of pleasure/pain) is not the slam -dunk argument some feel it is. There are societal and cultural considerations. Which is why, it seems to me, the last few seconds of Principle Doctrines talks about justice and living in a social setting.
https://www.bbcearth.com/news/seven-animals-who-mate-for-life
(Oh, and Godfrey , you're welcome to reconsider your after I added my PostScript
)