I'm speculating the Neil Patrick Harris character and Billy Magnussen character had some BDSM interactions in college with each other, and never talked about it since. NPH/Rodger who is in therapy is a little more self aware about who he's attracted to, what his kinks are, and all of the things he's unable to get from his failing marriage with Eve. When Magnussen is pushing him around, it's turning him on and he wants to roll with it.
On the other hand, Magnussen/Frank has very little self awareness, so he quickly laughs off the BDSM vibes, he initiated. He's terrified of the idea of being sexually attracted to guys, which is why he freaks out so much when he thinks Dillahunt/Brett is gay and never told him.
In general the scene is meant as an escalation, to further show the extent friend groups/society in general would fall apart, if everyone started sharing their secrets.