I think one of the hardest relationship things for me, as an intensely pragmatic and practical person, has been learning to let go of who's being irrational or not. If you feel a certain way, you feel that way. We're talking about emotions here, so maybe the distinction between irrational and rational is sometimes less meaningful than we make it out to be.
When husband and I decided that a poly marriage made more sense than monogamy, I didn't initially take into full account the emotional aspect. It was what I wanted because it made logical sense in a way monogamy never has. It turned out to be what he wanted too, so we went ahead with it. I know my husband to be a more emotional person than I am, so I made sure to reassure him that I'd always be available to talk through anything with him. When he made the same offer, I smiled and said thanks, knowing I'd be fine. After all, I'm the logical one.
Then he met a woman.
I was unprepared to be jealous. Intellectually, I knew he'd made a great step toward even greater happiness. I knew about compersion, and I waited to feel it, waited to be as thrilled for him as he'd professed himself to be for me in the same scenario. It didn't happen, at least not at first, and it was completely irrational, especially considering that he was openly thrilled for me when I made plans to see a man I know and hopefully explain our situation and see what happens (a whole other topic for a whole other thread).
It was the biggest shock of this whole process that I turned out to be the one coming to him saying, "I'm not sure I'm 100% comfortable, can we talk about this?" And we did. And then we talked about it some more. This is still new to us, but we're working on it - at this point I am OK with him seeing this woman, and we're even talking about the three of us getting together for drinks or dinner or something.
So I guess the bottom line is, if you don't express your emotions because you're afraid you're being irrational, that itself isn't terribly rational.