Yeah, I can see why it can feel strange (especially for small town clueless cop like Todd, or will putting up with it), but I've definitely seen situations play out like this IRL where friends who aren't attracted to one another have sex with other people in the same room, so it's not like I personally think it's unbelievable.
This is a scene where a lot of characters are letting loose to put their minds off of the stress of what's going on and just let their instincts lead them to a small patch of enjoyment that they rarely ever get to have.
(I'd also say it was more of two twosomes in one room than a 4some, and that one of the twosomes turns into somewhat of a threesome with will x kane x sam, but that's just semantics haha.)
-George