In terms of exposure and being able to "show off", relationships and sex go hand in hand, don't they? Sex just has a added layer of intimacy:
~With friends: "I sure love my girlfriend"
~with close friends: "And the sex is amazing!"
Maybe it happens for the same reasons... being able to share your joy with others, makes it even better... maybe it enhances your status, maybe it helps your self esteem, maybe your just comparing dick size? Regardless, it is joyful...
However, if you plan to be poly (or have really naughty sex) you have to be careful... very few people will act shocked or question you more, some even not react at all and then gossip about you. I personally think that nearly each person has done more "kinky" things in the bedroom than they pretend to be shocked about in public.
Just keep in mind that not everyone will be able to just share your happiness, many people have the need to judge critically... it is natural and, in my opinion, okay. You can dismiss something as unfit for yourself, for your own person. However, only in rare cases should one judge for others.
I and my partner(s) have choosen to expose ourself and face that critic, infact I do get many weird looks and questions (So you don't take it in the butt? No, I don't!). You have to know for yourself - and discuss with your partners, if your willing to expose yourself like this.