I have a poly relationship with my bf. First he said I could date women but not men (I am pan). Because then he would be jealous. Now I like a man and told my bf, we fought. I don't get it? Why would he be jealous of men and not women I asked him and he doesn't know. Do you have any idea?
First off, if your partner doesn't know how to talk about what's going on in their head, it's okay to ask them to do more introspective self-work. "I have this feeling that I'm going to ask you to act on, but I can't explain what it means or where it's coming from" is a non-starter as far as I'm concerned. This is what I mean when I'm always recommending introspection and self-work. Sitting with your thoughts and feelings, examining their contours, finding words for them - this is a requirement for healthy polyamory. If you're going to act as if your feelings are facts and let them dictate the terms of your reality and your relationships, you should have a solid understanding of what they mean and where they're coming from.
Your partner is asking for something called a One Penis Policy, or OPP. This is common in relationships where the 'primary' couple is a man and a woman, and the woman identifies as bi or pan. Men tend to be more jealous of, and feel more threatened by, other men. It often comes from internalized ideas that penis-in-vagina sex is "real" sex, and therefore two women having sex is somehow less "real," and therefore less threatening. It also comes from assumptions about partnerships, where "the man" is the one in control or in possession of "the woman." Therefore, another woman wouldn't be a threat, but another man would.
Even if your partner isn't overtly sexist or possessive, these assumptions are super common in our culture and worm their way into the way we think about reality. To most people, i's not an opinion or a worldview, it's just "how things are."
You and your partner should do some reading about OPPs (also called One Dick Rules, or ODRs) in polyamory. You can start here and here. Encourage him to find words for his feelings and start thinking about how he can explain them to you and work out how to manage them in the contect of your polyamorous relationship.
Of course, no one is obligated to, and many people are not able to, research and rationalize themselves out of their fears and feelings. Your partner may choose to just hold to this rule, citing his discomfort. He has the right to choose not to try and 'get over' his feelings, and he may try his hardest and still find it impossible. You can only control your choices: whether you want to stay in a relationship with these terms.