First, let's face the facts. You are frustrated. You get incredibly turned on by naughty talk about threesomes in bed, or watching porn with girl/girl or threesome scenes, but you're also scared. Scared that opening your bed (and legs) to another woman might mean you have to face your own greedy sexuality, and also that of your man's.
You want to share him, and yourself, with another woman, but some deep dark part of you trembles at the thought that he will fall in love with her or dump you for this other chick (our nightmares are always much worse than reality, so of course in nightmares she is 'perfect').
Let's acknowledge the fact that we are human, with frailties and faults aplenty. Men and women also tend to have more specific weaknesses in keeping with their sex. Most straight men have a fairly high sex drive, and I agree fully with anthropologist Helen Fisher that humans in general were not wired for monogamy.
I think the reason we see so much cheating going on in a society where 95 percent of Americans profess belief in God, means that something stronger is at work behind the scenes. That something is biology.
Without going into a long-winded academic discussion here, it boils down to the fact people are still very much part of nature. And nature breeds when it damn well pleases.
Set aside your fear for a moment, and just imagine. Imagine the delight in his eyes when you openly, without jealousy, share his beautiful cock with another woman. Imagine the intensity of his orgasm when you and she are lovingly pleasing each other and him as well.
Imagine your own orgasms (multiple perhaps) when she is sucking your nipples or clit in the oh-so-tender way only women instinctively know how to do. Imagine the love and excitement in his eyes as he watches you orgasm under her expert tongue.
There is no crash-course in overcoming jealousy to my knowledge, but focusing on the excitement, rather than the trepidation, is sure to ease the way. If it's a "first-time" situation with swinging or threesomes, my advice is to go ahead and give it a whirl – you will forever silence your own wondering this way. Most likely you'll love it and want to do it again soon. If you hate it, at least you can say you've tried.
The thing I always stress about everything is communication. Talk with your lover about how and why and when it will take place, long before you rush out to pick up a lady. If there are certain things that trigger your jealousy (like him kissing her in a certain way, or doing something special to her that you want only you two to share as a couple) then lay the ground rules before you lay the girl.
Also be sure and clarify what kind of experience you each crave/expect: Maybe you just want a swinging one-night stand, and he's envisioning moving in somebody to share your home and hearth. Or vice-versa. What a terrible misunderstanding that would be!
Remember how proud you felt as a kid the first time you shared a cherished toy? Well, you have that chance again. Decent guys are notoriously difficult to find (the best all seem to be taken) so if you have a great guy, why not reward him accordingly and spread a little sunshine to the other ladies who aren't so lucky?