I don't see anything at all wrong with them. I've been in a few, and they're a lot of fun for everyone involved. It really helps if you keep an open mind with it. It won't become weird if you don't make it weird.
That being said, there are certain rules that you should set up beforehand and abide by. It just depends on who you are and what you're into. I mean, if it's a MMF threesome, chances are there's gonna be another naked dude there. If that weirds you out, maybe you should rethink your participation.
Wait a minute, this is the Bore. I can't be serious about this here.
MMM threesomes are the way to go.
spoiler (click to show/hide)
:tauntaun :himu
Himu just likes to watch.
Mupepe, has a dude railed you while your wife watched?
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:drool
Mupepe, has a dude railed you while your wife watched?
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:drool
no way!
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she railed me while a dude watched