Sincerely, I’ve never thought much about it. It doesn’t seem like something that would happen to us.
But I have two friends who are a couple and have tried adding a third person to the relationship, but after some time this person ended breaking up with both of them. It was sad as any break up would be, but the two are still together.
There is no “need” to be filled. Having a girlfriend could be nice, but never during our time together I felt like I was missing out.