HUGE assumption you’re making about their sex life there, bud. She could be a fucking acrobat in bed.
She’s doing this at a house with hardly anyone else around. Doesn’t even look remotely like a party. Even if that weren’t the case, it’s not her fault that some fucking losers take dancing as an excuse to be shitty people. Getting too close to the “what was the rape victim wearing” argument here.
You didn’t mention the other option for him: give absolutely no shits what a bunch of random people think based off watching a girl have fun.
One more HUGE assumption here about some strangers’ sex life based on a 20-second gif.
I get wanting to defend the guy based on some other comments, but it could just be that’s he knows that reacting this way is funny or gets a laugh.