So I could really use some guidance here reddit. Me and my partner have been exclusively monogamous the past half year or so of our relationship. That was the agreed upon terms when we started dating one another all those months ago.

Fast forward to present day and my partner dropped the bombshell on me that she wants to have sexual relations with her best friend citing the inability to have her needs met for the time being because of the distance. Me and my partner are long distance and she gets a significant quantity of more time with her than she does with me.

Reading through threads, I've seen people discuss how a hard boundary some of you have is not to engage in those sorts of acts with friends. Is there a particular reason why?

This was all a bit of a shock for me and I've been doing my best to handle it and adjust accordingly because I love her so much and I desperately want to make things work. I'm open to the idea of it even if I'm not particularly thrilled about it as long as I can get to know said best friend a bit and make sure we're all on the same page and that it really IS just casual sex.

My partner assures me her feelings for her best friend don't diminish how she feels about me and that they're entirely separate. She's reassured me she still wants to marry and spend our forever together.

Now admittedly, I'm very much monogamous. She is my person and I don't need anyone else and I was under the impression when we started dating that that was the case for her too.

She reassured me that nothing would come of things between her and her best friend because she is ultimately only romantically interested in me and not her and that her interest in her best friend is purely sexual.

Now, here's where things get complicated. She later redacted it and said it's possible she might be polyamorous after all. The tricky part here being, I'm simply not. I'm okay with just having her and I don't want for more and her best friend in question is also polyamorous

This sparks fear in me that I might be left in the dust because they could end up simply being more compatible together.

Compound that with the fact that when I've brought up how I'd like to get to know the person I'm sharing her with a bit she's shelled up on me

To complicate things further, she isn't wanting to give ground in regards to that at all. Her best friend has expressed an unwillingness to talk with me and my girlfriend doesn't seem to believe that she owes me that time of day when she is essentially involving her in our relationship in my eyes.

If it was really just sex why does it feel like something is being kept from me? It feels less like she's asking for casual sex with her as opposed to an actual exclusive relationship with this girl.

I need advice, I'm drowning in my thoughts and I want to keep her but I'd be lying if I said it didn't feel like she was fighting me tooth and nail to keep the things I need to be okay with this arrangement from me