Can't swear in front of Robins, because they're kids. Can't swear in front of Alfred, because he gives you that look. Can't swear in front of criminals, because honestly it just does not fit The Vibe of being Batman and he is a man who commits to his aesthetic.
But at the end of the day, he can very wearily look Superman in the eye, and respond to an explanation of why they just had to stop the end of the world again with "What the fuck, Clark."
His other arguments for the r-rating are Steppenwolf cutting people in half and Cyborg being "not too happy with what's going on with his life before he meets the Justice League" and "tend[ing] to speak his mind." Also, in the quote he did literally say "drops an F-Bomb"
I genuinely can't fucking wait to see the snyder cut
That said
It’s gonna be hilarious in all the worst and wrong ways