My question is: does this advice also apply to employees of companies that your company does business with? To give a concrete example for my situation: one of the receptionists at the front desk of the office building my company rents office space in. They're not direct coworkers, but on the other hand personal problems might still end up affecting the company.
I wouldn't even say it's always a bad idea to date coworkers. It is a bad idea to date people that
- you have any sort of authority over
- have any sort of authority over you
- you work with closely in your day to day
Everyone else is, in my experience and opinion, fine. Many successful relationships start in the workplace, and many couples meet there first. The reasons that the advice not to date people you work with is usually given is restricted to two main points:
Dating superiors or subordinates is always a conflict of interest. You cannot act as an objective judge of your spouse when it comes to reviews, promotions and salary raises, and vice versa. Even if you somehow managed to mitigate that conflict of interest, the appearance of impropriety will still be there, so it is a bad idea.
Dating people you work with often and closely will be a severe problem if the relationship sours. That means one of you would have to quit or you'd both be miserable. Bad idea.
So it doesn't matter so much who is the employer of the person you are dating, but rather, what is the relationship you have with them. An office worker dating a receptionist in the building? Probably fine
A sales associate dating someone in the purchasing department of a customer? Very much not fine.