Men probably do that for the sake of what I call "mood shattering humor". Sex can be a very uncomfortable topic when spoken about seriously between men for too long. The subconscious starts fearing that unless a crude joke is thrown the conversation may naturally lead down a path that the whole group will not be able to return from.
God forbid nikkas get too comfortable sharing their sexual quirks and turn-offs and someone says:
"Well I really enjoy it when my girl puts her finger in my ass. Do any of you guys find that strangely stimulating as well?"
So part of talking about sex between dudes is not letting things get to that point. It's a very strange social dynamic to watch play out in real life. It gets amusing when you really see it happen for what it is; nikkas trying to share their experiences amongst each other without letting too much of their curiosity spill out their mouth.