Well, if you work in corporate, this kind of stuff gets drilled into you by HR. For instance, it’s a big no-no if either person is a subordinate in the relationship, i.e. you’re her boss or she’s your boss. This would just open up the company to all kinds of lawsuits. Also, if the relationship ends badly, and you find her crying in your boss’s office, that would be very, very bad. Further, this is how a lot of sexual harassment suits get filed. I could go on.
That being said, if you’re a married couple working for the same company, a lot of companies actually look upon it favorably, because that ensures “loyalty” to the company. Management would even make accommodations for it, such as having both of you work at the same location. But, if you’re just dating, you wouldn’t be entitled to this kind of “preferential treatment.”
Hope this helps.