Nobody cares if two engineers hook up, but it's a Very Bad Thing if your boss starts putting the moves on you. Relationships where one person holds real-world power over another are prone to exploitation and violation of consent.
It's happened probably 10s of thousands of times in the past few decades... and some of those couples are now happily married. I personally know of a boss who married his secretary and a professor who married a student.
I think the words "Very Bad Thing" are a little excessive. It's risky behavior.
It matters because if one of those people gets promoted it can break the policy.
They're dangerous to the firm and bad for the people's involved careers.
Best rule for everyone is no relationships between co-workers and if one happens both people are fired.
US work culture sometimes feels so incredibly alien when seen from the outside.
oh wait.
If person A doesn't want to have sex with person B and person B would be inordinately happy to have sex with person A, just about all ethical systems would say they shouldn't have sex.
If person A has money in savings and is clinically depressed and person B wants to take that money and go on a vacation that would make them very happy, just about all ethical systems would say, no, that's stealing.
One of the things functional societies do is to limit the amount that certain people can make themselves happy as a mechanism of limiting the amount to which other people get hurt. Banning workplace relationships because of their serious potential for hurt seems like a straightforward case of this.
Nobody the debate is not between "some restrictions" and "no restriction", the latter is very much a straw man. We are debating where in particular the line should be drawn. For any given behavior X there are trade offs across the entire spectrum from full restriction of X to no restriction of X and the latter extreme is almost always far from ideal but much better than the former extreme.