After being together 5 years and occasionally adding a third person, my partner and I played separately for the first time this weekend. As he put it, "It's only sex, what matters is that we came home to each other." I'm feeling pretty bad that I had sex with someone else. Is this a feeling I should be having or should I be looking at the situation with the same open mind he is? Maybe I'm just being jealous. I'm not sure.
Relationships are complicated. Open relationships are very complicated.
If open play (together) feels good to both of you and it has worked without any issues, perhaps that's the boundary that works best for you. If separate play (without the other present) feels bad, then be honest with your partner and let him know that it is not something that you enjoy.
Trying to figure out why you don't feel good about separate play is like trying to figure out why you don't like deep-fried Oreo cookies. Neither extramarital sex nor deep-fried Oreo cookies are essential to your health, so listen to your gut. If your gut is sick with sugar and grease or it's uncomfortable with sexual activity that you didn't enjoy, then avoid the source of your pain.
It's a different story all together if you have ceased to enjoy sex with your partner, or if you have ceased to enjoy eating anything at all Sex and nutrition are instrumental to healthy living. Cookie consumption and consensual outside play are designed to be pleasurable. If you do not find them so, then it makes no sense to engage in these activities.
If your partner is requesting an open relationship with separate play and you fear losing him if you do not go along, then your gut is probably full of fear about whether or not this relationship is going to work for you. At the end of the day you will make much greater strides by listening to your gut than most anything, or anyone else.
So I suggest you stop questioning why it's telling you what it is and simply heed what it's telling you.