You’ll Make Her Happy
The whole point of a relationship is to make each other happy and have a good time. It’s a give and take deal, and right now it would be the time to give. Maybe she’s had this fantasy for a long time, and now you’re the man who gets to help make it a reality. She might be so inclined to return the favor later on with acting out one of yours. Ask her what she really wants out of this. Setting up a sort of guideline for her expectations can make it easier to fulfill them.
It Could Be Good for You Too
If you have never had a three-way with another man being involved then you have no idea what it’s like. Essentially there’s a couple of different outcomes that a 2:1 ratio with men to women has: the two men don’t need to have any contact, they can have minimal contact (think double penetration for her), or they can be sexual with each other as well as her. The absolute best thing about this is that it’s not gay. It doesn’t matter if you get wood while the other guy’s there or if you enjoy it. You’re not gay just because you have a positive experience with your woman and another man.
You can always stop
Your girlfriend loves you, and if you’re feeling uncomfortable to keep going after you have started your lady sandwich, she won’t make you do it. Just be honest with the two of them and from the very beginning mention that you’re willing to try whatever you’re comfortable with but if you aren’t you’re going to say something. The same goes for all of them. Keep in mind that once you do stop the whole party, you’re probably going to be avoiding threesomes for quite some time, possibly not even getting a chance to get into that fantasy of yours at all. But if it really bothers you that much then it’s going to have to be worth the price