This article was interesting. The interviewer asked different faculty members from various institutions on their stance of professor-student relationships.
I'd have to agree that it is unethical and that there is a violation of personal trust between professor and student. I think a professor should only be a mentor, someone you can talk to on a professional, and sometimes, personal level. I think having dinner with a professor in a group situation is fine, but getting drinks together could be crossing the line.
I do know more so of grad students dating their students from their tutorials. I've always wondered why they do this? As a student, I don't think I could date a guy who was teaching me in a classroom and dictating my learning progression in the course of study. I think I'd feel uncomfortable discussing my thoughts on a novel or article with someone I'd been intimate with.
Anyway, would love to hear explanations on why grad students and professors date their students, and why it's ok.