Once I am done with the short-term scenario where I have influence over a person, such an expectation would impose notable cost/harm without really providing significant benefit. Just because it seems unfair to you not to do it doesn't mean its ethical to do it.
@kimball - I think it's relevant, as it shows there's a potential less-formal link between the two.
ie there could be more to the relationship than strict "Professor-Student" ties. I don't see a moral issue (IF the power unbalance is now gone - @vadim123's answer below is good), but thought I'd relate this: in my old school, a male teacher had a fling with a female student while she was still a student, and it was frowned upon but nothing more.
So just be sure that the rewards of the behaviour justify, to you, the costs of being seen (incorrectly in your view) as unethical by others.
The love of your life might be worth more to you than a idle few dates.(That said, personally I think the important issue here is that most people in a position to judge this kind of thing, don't judge this unethical, rather than the issue that even if they do it needn't matter.
-1 for misleadingly implying that the (supposedly) "lifelong" student-teacher relationship of a thesis advisor and their former advisee has any relevance whatsoever to the question of dating. If I am single, I reserve the right to pick off a member of society to be my mate.
There are other inaccuracies in your answer, but this one is simply nonsense.no, and even if she didn't break my heart it would be unethical for me to write her a LOR after dating her. The very notion that I must temporarily restrain myself from choosing such a relationship with the people whom I actively have direct authority over... The idea that anyone is permanently blacklisted from being a potential candidate, just because I have ever encountered that person in a class which I taught, is way too unfairly exclusionary. I remember a college class which was required for all students in the college.
In this case, society would not be right to expect a professor abide by this particular proposed restriction.
The protection gained is minimal to non-existent, and individual personal burden inflicted is significant.
So long as the professor-student relationship continues to exist, a potential power imbalance exists, and an ethical problem arises.
Whether your colleagues consider this a serious ethical problem is addressed by Pete Clark's answer.
A female teacher had a relationship with a male ex-student after he graduated, and she was immediately fired.