TL;DR - How can you argue boundaries in a way that isn't "my way or the highway"?

Hi all,

My partner (25F, bi) and I (26M) have been in an open relationship for the last year at her request (something she'd wanted to try with past partners, I was the first to consent). We agreed to casual connections with people, and I've had many since starting. She isn't a fan of apps, and so only just met a girl she's interested in in the last few months. This person's been very forward with her, and they regularly hook up. This is fine and within the limits of what we discussed. What I find more challenging to handle is the picnic dates, nude drawings, and other intimate things they do.

What's making me write this post now, is my partner's gotten very dressed up (she never wears heels or dresses femme around me) and is out to dinner with this girl. About 20 minutes after she'd initially left, she also came back (with her date, whom I'm anxious about seeing) with a vase of flowers she'd given her, and has left it in the apartment with me to look at.

We never discussed gifts, so I can't fault my partner, though I do feel her FWB really should've asked before doing this. I didn't want to spoil my partner's night, but seeing the flowers got my chest burning and I've held my tongue before. I ended up texting her to say it was a boundary for me, & she immediately responded & apologized, so all is well there. I do feel like her FWB doesn't consider me often though.

In thinking about boundaries, assuming there isn't past trauma surrounding them (& assuming your partner genuinely has your best interest & is someone you could grow in this way with), how do you logically defend them? "I don't want you to do x because it makes me uncomfortable" really should be enough (& it has been), but is it selfish to have hard boundaries, knowing there isn't a legitimate threat to your relationship? If it came to an argument over her wanting to continue doing all this stuff (which I'd probably end with proposing breaking up), all I could tell her is that I want intimacy like what they're doing to be special/unique to us, and that I feel I'm being replaced. What if my partner said I should grow beyond my insecurities? That'd be considered shitty to say, but how could I logically respond? I've mostly gotten over the fear of my partner having sex with other people, which in western society is a massively open-minded thing, and I haven't pulled the plug on the intimacy they've had, but I feel like I could only leave that discussion as a person who's afraid of something that, in a vacuum, isn't unethical, that I'm being small (if they actually were becoming romantically involved, that's another story).

Thanks a bunch for reading.