Others have named the standard points:
1) That many husbands try to get their wives to swing and
2) that those husbands equate their wives excitement about the fantasy with a desire for the reality.
Let me play devils advocate... How do you know YOU really want this? You've swung before, but with whom? Was it someone with whom you had a completely loving and devoted relationship, so you know what it would feel like to have a person who is THAT important to you, have sex with someone else? Sure, you may like the IDEA of getting to have sexual variety yourself, and you may get turned on by the IDEA of an MFM. But, are you really ready for the reality of your wife being penetrated as you might think?
And, let me share another reality with you. What if she does it, and really likes it? That can be very threatening. And, what if SHE likes it but YOU don't? I have seen myriad postings by the spouse (sometimes the woman) who say, "my husband/wife decided they don't enjoy swinging, but I want to keep doing it, what do I do?"
I can tell you from experience that it can be very threatening when you realize how sexually independent your wife is of you. So, be careful what you wish for.