You're not alone, pal.
We've been married 31 years and I've always been hot to see her get it with another guy (with me right in there too). All the things you describe, well, that's our fave. So you're not alone.
My theory has always been that the ladies have way more stamina than men. So, we like there to be more men then ladies. MFM, MMFFMM, MMMF, etc. Maybe if we had started when bi was big we'd feel differently, but when we started bi was pretty darn rare.
The other thing is that when you really love someone, their pleasure and happiness is essential to your own. Indeed, the more turned on she is, and the better a cum she has, the more turned on I am, and the better time I have. So my feelings are both altruistic and self-interested. More bang for the buck.
If you read the "singles in swinging" forums a lot of the discussion is about MFM and how to set them up. Plenty of good advice there. If you're both open and up-front, and you talk this all out, you should be able to turn fantasy into reality without too much trouble, and no risk to your relationship. Just go at the pace of the slowest partner, but keep smiling and keep facing towards where you want to go.
MFMs have added a tremendous amount to our sex lives, and given us memories we'll treasure forever. Highly Recommended.