Dude, it's not about oh, those poor students, it's about professionalism. Professors are employees. It'll eventually get out that Prof A is banging a student, which is not going to go over well with the other students. People will believe that the student is getting special favors or the professor might be seen as a scumbag who is taking advantage of a perceived power differential. It's like the rule that lawyers and clients, pastors and parishioners, counselors and clients , doctors and patients don't get romantically entangled- it's not about ruining people's sex lives, it's about being a professional, and understanding ethical issues.

This goes back to a theme that you have all noticed in my posts. Sex is great, but common sense is important too. Boning the hot undergrad in your colleague's Chem 101 class is way way more trouble than it's worth, and yes, sometimes we need policies for those who don't have the sense to think about what's going to happen after they finish.