I guess, in theory, there should be no "fear" of opening to a friend for more. If you truly are indeed that close you would expect that they would honor that part of you and that you were sharing more of yourself in a genuine manner.
But there seems to be this niggling fear that it will somehow change the dynamic. Now there is potential for sexual tension between you where before there was none. This if the attraction was not mutual.
Still - true friends SHOULD (in theory) be able to navigate such a bump together.
This is something that's bothered me for some time, as I do have a close friend that I've felt has had some attraction for my SO - but I can't verify that. She and I have talked about "possibilities" and I think there is some interest on her part. But because we are all very good friends - we've been reluctant to rock that boat.
I've had some minimal discussions over the years regarding my (our) outlook on relationships, monogamy etc - enough to let him know that our relationship does NOT follow convention. And left it at that. Anticipated questions - never came.
Have dropped various other little hints such as links to poly sites like lovingmore etc. No real reaction.
Humans are sooooooo weird lol