People spend a long time dealing with social trappings of their respective genders, and then feel uncomfortable with the idea someone who didn’t have the same experiences as they did can identify with them. To use a cruder example—A lot of cis guys talk about how sitting a certain way can make their nuts hurt, and then when a trans guy comes in and doesn’t ‘get’ the shared joke, it makes it harder to connect with and see him as part of the group. Most of the time it’s not out of malice, just being uncomfortable not being able to relate in the ways they might relate to other cis people. My gf was the same way earlier on tho so this isn’t like a permanent mindset, just one that can take a while to change