ISTM that he's predicting some very real social frictions, but that calling them "consequences of sexism" may be a bit silly-- unless you posit that literally any institutional dynamic that might disadvantage women in some way is per se structural/institutional sexism, which is really just a matter of semantics. They're consequences of forcing people presenting with very different gender roles (male vs. female) to interact in a newly diverse environment. This will always involve some compromising of values.
That it’s not clearly biological for a man to place a high priority on seeking a woman for “individual sexual or romantic gratification” in a way that can cause him to view other men as competitors at the expense of “group loyalties and shared destiny”?
If scientists of yore had "remain silent until you find a reason for your opinion" we'd probably still think the sun revolves around the earth.
Strictly speaking, as you suggest, it wouldn't be wrong to write, "I thought about it, and although I don't have any reason to believe it, the notion that women don't form groups for the sake of comradeship settled in to my mind, and with a gust of wind, floated from there to this paper." However nobody would ever write that! Passing off guesses as knowledge only works as a career if you try to dress up your guesses as knowledge, for example by phrasing them in a way that makes it sound like they're already commonly known.
What he wrote was essentially common knowledge at the time, not a novel theory that needed defending. That’s why elsewhere in this thread it is pointed out that you can’t judge it by today’s standards. We have different common knowledge today that people use as justifications for their opinions without feeling any need to cite a paper. Some of that knowledge is right and some wrong.
For a modern example, you could take a look at a comment that says his conclusions were wrong without citing any evidence.
> They may become inhibited and stilted, self-consciously muting the more overt expressions of their comraderie because they feel that frank vulgarity is inappropriate in the presence of females, even if that vulgarity is a male social lubricant and if women profess not to object.
They may openly rebuff her presence because they are unable to relate to her on masculine emotional terms.
They may treat her with patronizing tolerance, as the unit's mascot.
They may being to compete with each other for her attention, breaking up group loyalties and shared destiny for individual sexual or romantic gratification.
Although his conclusion may have been wrong, it sounds like he was ahead of his time in predicting some consequences of sexisim that many feminists point out exist today.
To balance out your opinion of how right he was, he also said this:
>In contrast, women do not naturally band together ritual comradeship.
Clearly, he didn't provide any sources or reason to believe that. His article was armchair speculation. Repeating cultural beliefs without support, as if they were scientific facts, served as the mainstay of "polite" racism and sexism for decades, and although polite society has moved on from those cultural beliefs, it's a lesson to all of us that the practice of writing these unscientific articles never went out of fashion.