In the English dub of the third episode, Pleakley mentions that his species is genderless. But in the original series, he is shown to have both a mom and a dad, and is even betrothed to a female of his species, and his mother asks him to start wearing men's clothing. If the species was genderless, then why would they have a culture with males and females, and why would his mother by bothered by her son wearing female's clothing when, if they were genderless, he could just as easily decide one day that he preferred to identify as female, or just continue to ride the line like he does now.
I've never seen the show, but I've got some ideas for answers. One: They Just Didn't Care. Two: Simple mistake caused by not having time to check. Three: Maybe things were changed between series to make that possible (I don't know what, though). Four: I don't know if it actually exists in this universe, and, depending on interprtation, could overlap with three, but...It's magic, I don't have to explain it.
The line was proberly put into Yuna and Stitch as a cop out allowing them to have a Pleakley be a cross dresser but not really. The need for this could be the change in target audiance or simply because they lost their old cop out. In the origional Lilo and Stitch Pleakley had a excuse to cross dress. He was in disquise. There was also decent reasons for him to choose a female disquise. He had three legs so local dresses that almost fit would be easier to find than pants he could pretend fit, and a dress could even hide his third leg. A wig could also hide his antena. Sure in the series he got a little too into character but he still has the reason. In Yuna and Stitch Jumba and Pleakley do not seem to take the maskerade nearly as seriously, therefore he has no reason to cross dress except that he likes it.
Why did Stitch run off so quickly when Lilo wasn't on the beach? At that point, he had not only been on Earth for years, but living in a first world country where if he and Lilo never had a cellphone, they would have certainly would have had one in her house. Calls to space might be expensive, but wouldn't they have kept in touch? Why didn't he just go to the last pay phone on the island call to see if she was just stuck in traffic? Or gone to her house to make sure she hadn't mistaken the date they were supposed to meet was the next week? It's implied that she was just delayed a few hours. Also, why wouldn't they try to figure out someway to communicate? It would have saved they so much pain if she had written a postcard going: "How's it doing? Nani's pregnant, and the baby's due around the time we're supposed to rendezvous. Don't worry if I'm late, just come to the house and meet us there, you know where I live."