It's a vague term. Exactly when someone is "dating" can't really be determined. Mere mutual romantic attraction does not necessarily mean they enter a relationship. Also, can you be dating if you don't go on dates? Often, comics imply more than that they explicitly state. Also, it's a modern term not fit for, for example, Brian Kent (New Earth) or Katherine Manser (New Earth).
It's a fleeting term. It may be used for a romantic interest in one series or storyline, but after that ends, there's no indication whether they're still dating. Which ties into...
It's not a legal term. The Census still calls all unmarried people single. Boyfriends/girlfriends do not have the same visitation, tax or benefits rights, and they're not legally recognized as relatives.
And probably more. We've had discussions about this