Slightly pedantic, but Keith's note says:

"With increasing frequency, we hung out, drank wine, and I helped prepare him for interviews with tech startups"

...so this case doesn't seem to involve women, not directly. Your observation still stands, though.

I think it is an acceptable loss for high ranking (founders, directors, officers) people to not recommend their sexual partners to their employers. If you are in a relationship, having diversity of income sources seems like a worthwhile secondary reason, but I'd kind of hate being an unrelated employee who had a founder's spouse as a coworker or manager, too.

It shouldn't be just legal concerns. I've come to observe that working with personally close people (girlfriends/relatives) is just bad judgement, period.

There will come times of interest conflict, and it will hurt either your business or your relations. If someone works with his girlfriend (or people hook up with someone from workplace) and get a painful breakup, it tends to poison the working environment for the whole team. If there are relatives on-board, then people will be angry with a feeling of nepotism no matter if it's there or not. It just doesn't work out.

So - don't encourage your GF/BF, spouse, mom or nephew to join your company, spread out. I know a couple of married couples that first hooked up in the workplace - but it all started to be nonproblematic only after one of them quit the company.