Thank you both for replying.
Okay, let's unpack this.
We are probably in a bit of a unique situation, where our third is my best friend and co-worker I see every day, 40 hours a week.
I've had a crush on her the whole time we’ve been friends (about 3 years). We are both bisexual. About two years ago, my husband and I started a fantasy where she was our third, and a few weeks ago it became a reality.
So, you've always wanted to be with this woman sexually, plus she's your best friend, you like her a lot, you're with her at work and outside of work.
We are trying to do a “friends with benefits” situation. Neither of us are “dating” her. But they've started texting very lengthy messages every day, all day long. The first week it didn’t bother me, but slowly it just made me uncomfortable and extremely jealous. We had always fantasized sexually… the bonding was unexpected.
If she's your "best friend," you must love her as a friend, right? And you're also sexually attracted to her. I don't know, but when I combine both of those aspects, it makes for a romantic adult relationship. It sounds like your husband also enjoys the sex, and is now enjoying talking to her a lot. He's getting into the same kind of emotional place as you are with her, right?
Why fight the love? Combine deep friendship and amazing sex, you get polyamory, not mere FWBs. This is a board for polyamory, where this kind of relationship is the whole point. We don't fight against loving multiple people in this way, and while we might struggle with our partners falling in love, it's hypocritical to try and prevent it.
I truly do trust them both. I’m not scared my husband will leave me. He’s made it abundantly clear I’m his primary. The physical aspect was absolutely incredible. I had no issues in the bedroom. My husband needs the friendship in order to be comfortable having sex with her. But I’m not sure how to come up with boundaries.
Typical boundaries:
Don't get carried away with NRE and neglect your original partner.
Turn off phones when you are meant to have quality time with original partner.
Work out a schedule for splitting time between partners.
Don't forget to date your original partner(s). (Sometimes the new shiny person gets all the focused romantic attention, and the established partner is the old shoe that you do chores with.)
Practice safer sex.
Come home when you say you will.
Take a shower between having sex with partners.
Communicate communicate communicate.
Be respectful!
Now, you two are attempting a triad. These are extremely difficult, as I said above, because the members of the couple tend to compete for the third's attention. This can get really ugly, because each dyad develops at its own pace. You may feel a bit like husband is stealing your best friend away from you, just as you've started having sex with her.
Each dyad needs time and space to be nurtured and grow. That means one-on-one dates, and one-on-one sex. You say you aren't "dating" friend, but personally, I think you should. You're trying to keep her as sort of a second-class citizen to protect the marriage. But this isn't swinging, you already really really like (love) her. She could become a co-primary and still not threaten or end your marriage. Many many polyamorous people end up with two primaries. (I have two, so does my gf. We each have our own separate bfs and we split time about equally between them.)
The dyads in your triad--
You and husband
You and friend
Husband and friend
The bonding your husband and friend are enjoying is a natural hormonal outgrowth of the sex. Our bodies just do this. We have sex, we fire oxytocin. It's the "bonding hormone." It actually affects the body like a drug, making us feel high, warm and glowy. You can't do away with it. You can work to limit and control it by thinking with your big brain in your head, not your little brain down below, if you know what I mean.
