What's your opinion on "genderfluidity"?

Don't agree with it or believe that it has enough scientific backing to be proven, but I will try to be nice to people who believe they are genderfluid. I once lost a friend over my opinion on this and they thought my opinion shouldn't be accepted and when I said we should be able to discuss our opinions they were like 'it's not an opinion'. 😓 Goes to show that people on both sides of the debate can be just as bad as each other.