I'm afraid you've got me there. Still, I'll try and justify myself.
Firstly, his befriending of women. Personally, I put it down to the fact that he's the protagonist of a kid's show and therefore suffers from being too likeable (notwithstanding his arrogance at the beginning of the "saga"). He must befriend everyone because that's how he's written - child-friendly and without enough flaws to make him appear human enough, but enough to show him as a teenager.
Secondly, his 5 closest friends. Now, I'm not a great fan of the Pokemon anime, so I won't pretend to know all of the people you listed there. However, I did follow it for quite a while, so I'll take a guess. Misty and May were both main protagonists, much like Ash. They featured in every episode for the series' they starred in, were involved in all of Ash's escapades and overall share the limelight. However, despite the purported romance, it does boil down to the fact that Ash is, in essence, a kid. A young teenager. In the early seasons, he would have no real reason to have a relationship with women, aside from a friendship. However, as he grows older, throughout the progession of the show, obviously (assuming he's human) he would start to fall in love, correct?
Writers can't have that.
Why? Because it wouldn't fit. Pokemon is aimed at children (and no offence to yourself) who are young. Let's say, around 7-10 years old. How many 7-10 year olds want to sit and watch a drama about a young boy who slowly matures, and falls in love with his traveling companion, and best (human) friend? Or would they rather watch a show about a kid, seemingly about their age, give or take (since there's no mention of his age other than the opening episode) who has monster friends who he battles other like-minded people with, to become the best Trainer ever?
There's the nub of the problem. I'm not saying Ash shouldn't have an obvious in-show relationship. In fact, I reckon it'd add to his character. However, the target audience prevents it. Sure, the writers can put in subtle hints, and have kisses on the cheek, and starry eyes, but then it'd detract from the basic premise of the show - monsters beat up more monsters.