Yes, one can be both an economist and an environmentalist.  Indeed, environmental economics is a growing subfield within the economics profession (and Marietta College offers a course on the topic).  For more information, check out the Association for Environmental and Resource Economists at http://www.aere.org/. 

I do value student input as part of our class discussions.  One thing that I try to focus on in class discussions  is whether student arguments are based on positive or normative analysis.  I primarily want students to focus on positive analysis--statements involving the use of theory to make testable hypotheses.  Such statements generally exhibit either correct or incorrect reasoning and it's my job to point potential flaws in reasoning out to students.   Statements involving normative issues, however, generally involve someone's opinion on a matter--and there are no correct answers in these instances.   I think students have difficulty distinguishing between the two types of arguments and I don't want to engage in lengthy normative discussions so early in the semester without formally developing our economic models.