It has certainly been interesting for me as well. This is a new experience for me, a partner having a new interest. While I have been non-monogamous for the past four years, Alec hasn't had any activity with others during that time, and neither has Mya since we've been together (besides JJ but he was already there in a different way).
To a very large extent I am feeling good. I totally enjoy compersion. I feel happy that Mya is happy, and I feel glad about the comfort and enjoyment she is getting with Bob.
There is a tiny feeling of strangeness, though. It doesn't take away from my wholehearted endorsement or their relationship, whatever is included in it. It's not really discomfort, more like a bit of restlessness.
Since I always like over-analysing stuff, that is what I will proceed to do. I just don't want it to cloud the fact that I am feeling mostly happy and calm.
I think the feeling of strangeness comes from a few places.
Firstly, it's just the new experience of partner having a new sexual(/potentially romantic) interest. I feel fine with that, it's simply new. It's something concrete, a potential change for her in her life. I have spent so much time thinking about how I might feel when this happens, and me and Mya have spent time talking about Bob. Yet, it's almost surprising to finally have the experience. It's not bad at all.
Secondly, there's a bit of ego. After all, I was the new and shiny with Mya just a while ago. And now I'm no longer quite as new (while still perhaps a bit shiny). Oh noes.
There's somebody else she's experiencing new things with. And I do want that. I completely wish good, enjoyable experiences for her whether with me or not. It was just some ego stroking to be the only new and shiny for a while.
Thirdly, there's a tiny bit of envy. Bob is there close to Mya and I'm here, and I have to wait for my hot sex with her. This feeling is a familiar one, though, since the sex with Bob is simply a reminder that I'd like to have sex with Mya but can't right now. In an LDR, that feeling happens from time to time, whether there are specific reminders or not.
New is sometimes a bit strange, but that will pass. Ego stroking is something I can certainly live without.
And I have made peace with our distance a long time ago, so that's no biggie. And it is fun to see Mya all happy and gloaty (and get to tease her a bit).