As a reader of all of the above genres, I will say that there are noticeable differences in what is categorized as yaoi vs shounen ai, yuri vs shoujo ai. It's a bit difficult to explain without reducing the nuances to simply "porn/hentai" and "not porn/hentai," although broadly speaking, yes, yaoi/yuri are much more explicit. There have been works considered as shounen ai/shoujo ai that, albeit having sex and sexual content in them, aren't considered yaoi/yuri. An example would be the classic Kaze to Ki no Uta, a founder of the shounen ai gender and considered purely as such (on MU, Wikipedia, etc.) despite it containing depictions of "racism, homophobia, pedophilia, rape, and drug abuse." As the focus was not purely on sex/sexual acts or eroticism despite them being elements of the story, focusing instead on the tragic events that happen in relationships involving people of the same gender, it's classified in both Japan and in the Western community as shounen ai, not yaoi. On the other hand, there are yaoi/yuri works that don't necessarily have explicit sex in them either but aren't considered shounen ai/shoujo ai. Think of the currently wildly popular Citrus - I haven't read it in a while, but I don't believe the two main characters have had sex yet (although there's been covered up nudity and erotic kissing and all). Despite that, it's been considered from the start as firmly yuri and not shoujo ai. For many readers, it's because the focus is on the drama and the erotic acts between the two that it's labelled as such. And lastly, there are works depicting romances between people, main characters even, of the same gender despite not being considered yaoi/yuri/shounen ai/shoujo ai. Examples here are titles such as Revolutionary Girl Utena, Yuri on Ice!!! (yes, yes, an anime), Antique Bakery, or The Rose of Versailles.
Therefore, I think that, even though the technically correct translations so to speak would imply that there is much too similarity between shoujo ai/yuri and shounen ai/yaoi, to the scanlation communities upon which this website, MU, and tons of others have grown from, they are regarded as vastly different things and so should be treated as such. Do we (speaking as a primarily Western-based community) use the terms "incorrectly"? Probably, yes. But, in doing so, have we not grown to attach different meanings to the terms than were originally intended? Yes. The "misuse" of the term could be likened to the usage of "otaku" is the Western community vs. in Japan, or whatever other terms we've adapted over the years to serve our own purposes rather than what they meant in their original language. And now, they serve a practical function in being defined differently in this community, despite them being perhaps used interchangeably or having different nuances in the Japanese community. They're useful to us here in this community in this sense, and just because there's overlap between yuri/shoujo ai and yaoi/shounen ai, doesn't mean they should be combined.