I totally get that. It was very hard to have that first conversation, and I brought it as "I've had this fantasy about being with you and another man." Fortunately, he thought that was hot and not threatening, and we had sex whispering in each other's ears about what that would be like. The next day he told me we could really do that if I wanted to, with the caveat that we also play with a woman. Then we talked a whole lot more about what we would really want out of doing this. How it would enhance our relationship, etc. That worked great until I found myself in love with someone else, which was initially not the plan. Then we had to recalibrate again. It's definitely harder to open up and change the relationship paradigm than it is to go into a relationship with no expectation of exclusivity. Next life, I'm going to be poly right from the outset...lol!
Some couples feel more comfortable as swingers, because then it's really mostly about sex, and there are ample opportunities to be together while involved with others, depending on what you agree to do. say at a club or house party, rather than going off to separate rooms (although some swingers do that). Even though we are now poly, James and I still only see others together, although that might change if I ever get an opportunity to visit my long distance partner. The good thing is, the two of you get to decide (if you choose to go in this direction) what the parameters are for the two of you.
Not trying to push you to pursue this but one way to approach the topic with little risk would be to watch a movie or show that has poly or swinging characters and then just ask her what she thinks about it. Could she ever imagine doing that, and that kind of thing. You can gauge a lot about how receptive she'd be to talking about it for the two of you from that. Two thirds of monogamous relationships that open up are done at the suggestion of women, so you might be surprised. But, you definitely don't want it to sound like you're bored or missing anything, because that couldn't help but bring up bad feelings. It really has to come from a place of wanting to enhance and expand on what you have and to make that very clear.
Here's something I wrote a while back about that.
