Professionals do in fact date coworkers. It happens all the time - it's a matter of culture. Spend some time at Microsoft, for example. "Microsoft Married" is in the corporate slang lexicon on the intranet.
He's implying that if you do date coworkers at the office it's unprofessional, not that people in a professional capacity don't make bad decisions and put their career at risk by dating coworkers.
Of course, what he's managing to imply is that women should not date men at their offices. Because it victimizes the men, who are forced to retaliate when the relationship ends, and then they look bad in the press.
Could you be more paternalistic? We're talking about a woman who was a partner at one of the best VC firms in the world. Pointing our her unprofessional behavior implies nothing more than that _she_ should have known better. If you really want to be a white knight, stop treating women at the top of the industry like interns.
"White knight" is definitely a term that makes me sit up and take notice that maybe it's me that might be on the worst possible side of a debate.