tormontreal; came across a great example post just now of why, when 'organic' situations arising with friends or whatever you when the second male in the MFM is not a swinger, sometimes do not work out very well.
Observe this post. Swinging males that have been in the lifestyle a while have fewer problem in this regard. They've "been there, done that" and it's not an issue.
So, even if an 'organic' situation develops, and may be considerably less than satisfying.
Just some food for thought. I would not advise using this to pressure your girlfriend
Just communicate.