What do you think the difference between shoujo ai and yuri and shounen ai and yaoi is?

A while ago, I came to understand that Shojo Ai (Girls Love) and Shounen Ai (Boys Love) were about same sex romance stories, while Yuri and Yaoi were similar but more explicit and graphic in nature. But nowadays, I prefer to just drop the distinction and use the terms Yuri and Yaoi for any same-sex themed animes.

I'm noting that Media Blasters have a label called "Yuri Fan" (and might have one called Yaoi Fan). Most of these are hardly what I'd call explicit. Strawberry Panic has only a limited amount of fan service beyond an occasional kiss, for instance. And they don't have an "18+" rating. If those anime are Yuri, I can't imagine anything "less explicit" than them yet still same sex that they would be labeled as Shojo Ai instead. So I don't think the international anime industry is supporting the distinction between the terms. And then there's Yuru Yuri. It's got Yuri in the title, so I'm gonna call it a Yuri anime, even if it's mostly gag material and not really explicit, and even if they're just referring to "Lilly"s in the title instead of Yuri's other meaning.

Maybe there's room to make a distinction between Shojo Ai/Shounen Ai and Yuri/Yaoi in the realm of manga, doujinshi, and fan art, but in the realm of anime, I don't see any need to distinguish between the two. If there was a more explicit yuri title than normal, I'd just call it a yuri ecchi or yuri hentai.

There's probably no right or wrong answer to this, as they're terms that may have different meanings or connotations to different people in different contexts and evolve over time.