How could they do that????

I never understood why women allow their SO to check out other women in front of them or even do it together! My sister in law does is, she’ll tell my brother “hey babe look at that girls ass” and it’s everywhere they go. She says she’s confident but ummm I am too. I just feel it’s weird and disrespectful. I would never do that! I get we’re human and we all tend to look every now and then but if I caught my man doing that shit right in front of me I’d be pissed. I would like to think that I’m the only woman he’s looking at. And why do you ask that I bring this up? I’m just curious to know why some women allow it? Help me understand.