BF becomes Friends w/MY friend

I have an issue with boy/girlfriends who becomes good friends with their boy/girlfriends friends.
In my perspective there's always such high risk for misunderstandings, someone who get's left out, hurt, talk behinds someones back and much more. It's not a matter of trust but of having your own life, your own friends beside your life with you loved one.
Obviously boy/girlfriends and your friends should be able to acknowledge each other and have a small friendly chat. But allowing it to go further, I don't see how that could be okay.

Am I naive in this thinking...or even wrong?

My boyfriend has become good friends with one of my friends, one I don't really trust because of her selfishness and ignorance of others. They have started to hang out and they keep this behind my back. I know that they aren't doing much more than being friends (because they claim to have no attraction to each other and I have seen my BF being repulsed by my friend. Even more weird that he would like to be friends with her because of that but yeah...), but I don't understand how they think it's okay for them to behave this way. They keep their friendship behind my back and that even though I've confronted them with how I feel about them being friends.
They reply with the opinion that they don't agree and that my opinion s crap. In their thinking I forbade their friendship when I was only expressing how much it hurt when they hanged out without me knowing and did not respect my opinions and talked to me about it, just ignored it.
I wasn't or aren't invited to do things with them. I get ignored by them....Still I am questioning myself... I don't think it's okay but perhaps I need to suck it up? Maybe it's normal for boy/girlfriends to be friends with your friends, more than just acknowledge.

Anybody have an opinion?
I am so very confused.

I had similar worries about a friend becoming inappropriately friendly with my husband. Nothing to seriously worry about because I trust my husband but it just seemed off. It got to a point where she was hanging out with him more than me. When I expressed my concern to her, she told me that she was going to be friends with him no matter how I felt. That really pissed me off and we argued over it but didn't get anywhere. Then, she decided at Christmas time it was completely appropriate to take my husband underwear shopping because he wasn't sure what to get me. Nice gesture? Not when she bought herself underwear at the same time and the item I received was not what I had pointed out to her saying I liked, on a previous occasion. It was the item she preferred. That and a lot of other things ended that friend ship before the end of January.