When I was in high school, a number of girls sued to be allowed on the men’s football and wrestling team. The schools were advised to allow them, even if they got hurt.
Honestly, I see this the same way.
Those women were putting themselves at increased risk of injury.
Allowing boys to play on girls' teams puts the other players at increased risk.
It is for one to voluntarily accept additional risk upon oneself - it is not okay to force additional risk on others.
It's completely different. Aside from the risk of injury, the rare girl who wants to play with the boys is usually volunteering to spend most of her time on the bench.
It's an odd choice, but (as the father of two soccer playing daughters) I know of several girls who chose to stay with coed club teams rather than playing on a girls team. The rationale seems to be that they will elevate their skills as much or more playing coed rec, as opposed to switching to girls travel. As a practical matter, I imagine they got attached to their teams and teammates at ages 9-12, like their guy pals, and don't want to bust up the gang. Or, in some communities, there may not be an equivalent sports league for the girls, or maybe a girl just loves football.
In any event, the odd girl playing on a coed or boys team (which then becomes a coed team) isn't going to wreck the competitive dynamic. But the reverse is not true. Boys in any numbers playing on girls teams will ruin the game for the girls. If you value girls/womens sports, you have to oppose the gender benders.