My late husband had a friend who hated the fact he married me. I never did one thing sideways to this guy yet I "stole his boy", "robbed him of his drinking buddy". blah blah blah
Yeah that went well. My husband, never having been in that position, didn't deal with it very well which caused tension. He needed to check his friend.
FF his friend made it glaringly apparent he was a liability (used our home for an escape from an angry drug dealer), brought nothing but drama to our relationship so my husband cut ties when the final straw was his friend was passed out on our living room floor with him locked out.
Guess who wound up being the "bad guy" in all this? LOL That's right, me.

Fine, treat me like the bad guy, I'll be that thing, and I was, even in death. The guy was such a tool to me he was barred from the funeral.
Now I should be clear in that I denied my late husband nothing, ever, and even offered to drive him to see this old "friend", asked him repeatedly if he wanted to see "so & so" , he said NO. His choice.
Why I was expected to put up with this guy in death is beyond me. He was never around, never owned his behavior when we were around him.

My point in all this is your "friend" has a hand in this also. Being able to be open, transparent & putting people at ease about "friendships" or former love interests is a gift that not everyone,and I'd say very few know how to do well when it comes to new chapters in life with a budding love interest.
Some even tho nothing nefarious afoot just make things awkward . If there's even a hint of secrecy it can put the new relationship in jeopardy.

You mentioned elsewhere not subscribing to "hierarchy" of relationships, well that is just not reality.
There is a hierarchy in all things, it's just nature. You've been demoted for lack of a better term and don't like it, because of your friend wanting to protect his newfound situation.
Distance being a liability. A bird in the hand, vs in the bush. Proximity even with truly platonic relationships is a factor in the sustainability in any relationship.

It sucks yes. I had a dear friend I considered a soul mate. We had a pact made when we were in high school that if we ever found ourselves single and alone when older, that we would marry each other.
We could go a year or more having lost touch, and then find one another again & talk as if no time had passed. We loved each other so much we didn't interfere with one another's current relationships, but knew we'd always love one another. Of course he died before we could see our pact to any sort of fruition, but such is life. And I do consider our bond a once in a lifetime rarity.

Yes, I actually have one female friend with whom I've remained friends, to some extent, after she was married.

Now, how is it an infinity promise ring, if it's just a temporary place holder?

I've experienced the general problem that people tend to move in "packs" -- and those tend to be roving gangs of singles or couples. I suspect it has its origins, largely subconscious, in the notion that "couples activities" precludes extramarital temptations. The divorced and widowed don't fall neatly into these categories.

During the interregnum between my 2nd and 3rd marriages, I became friends with a guy who was sort of the informal head of a group of guys who got together each morning for coffee. He had no problem inviting me to dinner at his house with his wife and occasionally his adult children. We occasionally went to movies both the two of us and with his wife. It is the only exception I ever experienced, to the general rule that once you're married, single friends feel like awkward 5th wheels around you, and once you're not married, married friends seem to lose interest.

I guess the "bridge" between those worlds in this case was the group of guys, and the fact that he was a retired captain of industry, accustomed to bringing colleagues home and his wife was accustomed to entertaining them. Anyway, he seemed to completely lose interest in staying in contact, when I moved away, so our friendship was one of convenience anyway. And this would not have worked if we weren't the same gender -- or if I had started doing things alone with his wife

Real best friends are best friends forever and understand that a new partner for either doesn’t and won’t interfere with that friendship.

Not wise to allow one's mate to be "friends" with women.
Most cis hetero men are only friends with women because they are hoping to get lucky some day, no matter what the women think.

I would dump, or never date in the first place, a man who hangs with women friends and looks to them for fellowship instead of me. Also men who cling to their moms, sisters, or exes.

To each their own ...

Life sheds friends . I can't remember this specific situation for me , but have heard it for others