Some objections:
1. The Boy Scouts have always been a boys' space.
False. "Scouting" as an organization across the world has been co-ed for decades, the US is lagging behind in this area. Even when it was first generated, girls were not dissuaded from Scouting. The segregation implemented then was simply an extension of the general gender segregation of society, nothing to do with scouts itself.
Where they can be themselves without being sh*tted on by society for it.
False. Boys of that age are generally socializing with other boys their age anyway. Any social criticism and judgement they receive is going to be by their male peers which doesn't go away in Scouts. Scouts provides no insulation for this.
If girls are allowed in, eventually you'll have woman scoutmasters who'll approach scouting from a woman's perspective, which typically means harshly prohibiting any and all rough play, which can be good and healthy for boys and also a lot of fun depending on how it's done.
Baseless speculation.
2. Many such guys, being still young and dominated by their hormones, will excessively try to attract the attention of a female scout.
Dubious in theory and disproven in practice where co-ed scouting already exists.
Some of them will cross the line into harassment or even worse. At this point, it'll be reported on by the media, and the response, of course, will be to sh*t on boys for being their natural selves in what's supposed to be a boys' space.
False. Harassing girls (or worse) is not boys "being their natural selves." Also false that Scouts is "supposed to be a boy's space." As stated, the ideals and purpose of Scouting is gender neutral. The gender segregation (also racial segregation) was simply a result of the segregation of general society, not a mechanism implementing by Scouting.
3. Related to the second point, many such guys will be distracted by a pretty girl and not take scouting as seriously.
False. Not a problem in practice where it exists, or in any other co-ed environment (such as school).
If the response to this, of course, is to blame the boys in question for being distractable, then once again they'll be sh*tting on boys for being their natural selves in what's supposed to be a boys' space.
Scouting isn't "supposed to be a boys' space."