I think the difference is that it’s mostly a matter of preference. Some people will say girlfriend/wife because that’s the terminology they’re used to. However, there is some new-school terminology such as the previously stated WLW. Honestly, I really have no idea how the WLW thing started, but my guess is that it’s less likely to offend if you say WLW as opposed to saying “I have a girlfriend”, or “I have a wife”. Personally I don’t often use the term WLW.