I had a ethics teacher ask about this idea. I think there was a two year rule in the social worker's ethical principles although some argue that once a client always a client.
I stated that I could see doing it if the therepeutic relationship was short-term. I used the example of a police officer having a few sessions after being forced to shot someone. Then, the two meet again three years later or so. If the relationship was short-term and superficial (not in depth therapy), I could see someone marrying a client. Another example for me, would be if you were a member of support group/group therapy for brief time and then married the person. I would have trouble enforcing the idea of never being able to date.
However, if the relationship is long-term and/or in-depth, I am opposed to marrying/dating. I doubt that I would ever relate to my current T quite like I do other people because of what she knows about me and transferance.