In this case and this one alone (because this thread is dedicated to it) I believe they behaved as they did because they thought they could without consequence.
Like many pps I went to an independent school with a strong rugby culture albeit in the ROI. Both my brother and father played and still love the sport. However as neither ever drank much, they never really participated in the more laddish post match socialising that seems to be part of the culture. That’s not to say they weren’t aware of it and they were pretty explicit in warning me off. To a greater extent though I was probably protected from predatory behaviour because I was x’s sister/daughter - the only upside to a crummy patrician society. Not all girls/women were given the same respect.
The way these “top shaggers” referred to women in the WhatsApp - brassers, Belfast sluts - is not how they’d reference each other’s sisters. It’s language reserved for those they consider fair game.