I think there are possible issues around sexualisation and objectification here. Are arses not ignorable? All arses? Or just sexualised arses?
If it's just sexy women's arses that you are talking about as something to take your mind off everything else. Then you probably have a shallow mind, and you are not giving any thought to how that might make other people feel. I could see how articulating that might make someone feel uncomfortable and in company that they didn't want to be in.
Like singing tits fanny and forest.
... And like singing that I would defend your right to say that. Because it's not against societies rules, and nor is the intent aggressive or threatening.
Which doesn't mean that we can't talk about it, and perhaps consider the appropriateness of sexualising strangers. I'm pretty sure that shades can do that with a modicum of charm and genuine consideration of boundaries, without making them fear that they might end up in a bin bag at the bottom of a river. As I'm genuinely sure that some people can't. Often really transparently so.
I find this sexualising randoms very strange. I also know it's quite unusual to feel like that (and not just because you all letch at actors and pedestrians in the spring that you don't know at all). Women are on to it. It's why they might overreact to mild misogyny, when more pithy examples are less safe to react to.
So a swing dancing specky uni nerd duo might be a safer outlet for the frustration from a lifetime of fear and oppression.
I can feel these things, and yet I also would have responded exactly as I said. It's not binary right and wrong, and there is an equal right to responding to the level and appropriateness of the challenge to you.
Life is complicated. If we don't embrace that and have conversations and empathy about it, and different peoples expectations and values, then we are going to set a really shit bunch of binary rules that only ever cause us all problems.
