Someone who believes in cold-blooded mammals is either misusing the term "mammal" or the term "cold-blooded" or both, and I don't think I'd refer to "cold-blooded mammals" without addressing the question of where that misunderstanding is. If people don't understand you when you say "selfish" (because I think you are using an unpopular definition, if nothing else) why don't you leave it out or try another word? If I was talking to someone who insisted that mammals were cold-blooded because they thought "warm" was synonymous with "water boils at this temperature" or something, I'd probably first try to correct them - which you seem to have attempted for "selfish" with mixed results - and then give up and switch to "endothermic".