"I play football (informally) against girls (I’m a man) and I’m not a threat to their safety because I’m not interested in hurting anyone. I’m interested in us enjoying ourselves."
I can absolutely 100% guarantee you that the girls will have a different perspective on this.
I was once the girl forced to budge up and include the males who just wanted to play "for fun" - males who did not have the slightest idea how domineering they were and how their physical strength was way above ours. In situations where you would have attempted to tackle a girl you would hold back tackling the male as you would always come off worse and
Regardless of what your intentions are you aren't actually the one at risk of getting seriously injured, albeit accidentally, and having to adjust he way you play to protect yourself.
It's a lot easier for the girls just to stop playing.