Normally, if the firm is aware of your relationship, they will try not put you in the same department. This is what happen when my friend's fiance transferred into the firm we worked in a few years ago.

And if they find out later, high chance they will move you around to a different team.

BF/GF is hard to "see" and there is no real relationship on paper. So its harder to draw the line when it comes to setting ppl to work. You can't keep shifting ppl around because of their relationships, and you can't hire/sack people due to change of relationship status. Management will need to manage the couple's performence. From what I have seen in the firm I worked in previously, management dun care about bf/gf relationships unless its a superior-subordinate issue. Peer-to-peer is not that big an issue.

Husband and wife, and relatives are more clear cut, and usually they don't work together. There are incidences where husband and wife are in the same team/department. But in those cases I have not seem them working on the same engagement.