And it would not bother me if they were just to assert "I'm a fundamentalist / a fascist / whatever". They would have to be making arguments for their position. ,,, ...perhaps I view it as a sort of attack. If fundamentalism is true, then atheism is not true, and I am stupid or at least a very bad truth-seeker for being an atheist.

How do you feel when people make just as bad arguments for positions you agree with? Some possibilities:

Not as bad - you feel associated with the groups under discussion in these arguments, and much of your feeling comes from that.

Worse - you feel tribally aligned with them, and feel stupid by association.

Same - Just right, and your negative reaction is due to the bad forms of the arguments. Alternatively, the two above identities of feeling aligned to the speaker and a member of the group criticized are balanced. Test this by altering your level of kinship with the speaker, such as by pretending you also follow the same sports team.