Unfortunately, this appears to be so for some professors. I guess it is just another example of today's mindset where if it feels good it must be good, and how dare anyone judge otherwise.
Even when I was single and only an academic advisor at Ohio State, I never sought a relationship with any of my students even though there were plenty of attractive girls with which I would have liked to. Visual admiration is as far as I would take it. There is simply too great a risk for problems and even the appearance of favoritism can ruin one's professional reputation.
However, if the student and professor do not have a working relationship, then I don't see as much of a problem (e.g. a business student having a relationship with a Classics professor), especially when students are at the graduate level and the age difference may not be that great.