We have had a couple of MFM's and the first 2 or 3 were NOT with penetration as she wasn't ready for it. she had a better time each time and now loves any swinging.
Any guy in his right mind would be VERY happy to join in on that kinda scene.
on our first FMF she wasn't sure she wanted me to penetrate which i was fine with so we didnt. then she realized how fun and sexy it is to see your loved one having a great time. now she encourages it..
keep talking and make sure to follow the rules to a "T". then the rules might get a little looser cause she will trust you more and know how she is gonna feel in those situations.