If you sign a employee agreement/contract/company code of conduct you're agreeing to those terms.
Bingo. My employer not only has an annual training course on this but also requires you to disclose relationships such as a spouse, relative, etc. that works for the company too. Basically as long as there is not a conflict of interest where you have a manager/direct report relationship or 1 of your job titles has some kind of oversight or decision making over what the other person's job role is there is no conflict of interest and you are free to date or have a relationship with other employees. Would assume most companies have something similar for a policy too. It's there to protect the company from lawsuits and probably yourself as well.
Guessing whoever Udoka was involved with there probably was some policy that considered it a conflict of interest or possibly required it to be disclosed to be compliant with their HR policies. Maybe more to the story than that too that makes this more complicated but if he knew this relationship was against his employer's policy then they have every right to suspend or terminate him.