SHAMPOO

First, I'll touch upon the reasons I feel Shampoo is a terrible choice. For one thing, she comes from a village and a culture where men are treated like second-class citizens—and this is Ranma we're talking about—Ranma, whose name literally means "wild horse". There is no way he would ever willingly marry into that kind of matriarchal society and be happy there. He'd run away in a heartbeat, or piss off one of the elders and get himself banished, divorced, or killed. And that's not even considering the fact that his children would be raised as Amazons, in China, in a culture he doesn't understand, respect, or even like.

And secondly: On the most basic level, Ranma and Shampoo are just not compatible. She turns into a *cat* for one thing, which Ranma pathologically fears; and for another, she's manipulative, clingy, cruel, and has no qualms against killing. Ranma, at his most basic, is a protector, not a killer. I just can't see him adapting to a life where a motto like "obstacles are for killing" is thrown around with aplomb. Not to mention her forwardness makes him uncomfortable: Everytime she grabs him, he looks like he wants to run away and hide! What Ranma really needs is someone who complements him, who can meet him as an equal, and help balance his ego: Shampoo is too strong-willed, opinionated, and set in her ways for any of that.

Third, Ranma owes her nothing. He's not an Amazon so he has no reason to abide by their laws. Especially since her "husband" could have been anyone since Ranma wasn't even at his strongest when he defeated her. It was an accident. If they'd been at his school rather than the Tendos, she could have easily ended up engaged to Ryoga or Kuno instead. So it's not the strongest basis for a relationship to begin with. Especially since Ranma is already honor-bound to marry Akane, so there's no way he'd risk dishonor, or losing the dojo by accepting Shampoo instead. Ever since he was a child, Ranma's expected to take over the dojo, so why would he give it all away to follow a girl he can't stand, to go live in a foreign country and be used as breeding stock?! It makes no sense given his personality. He's not Ataru.

And finally, Takahashi-sensei has already done the exotic-beauty-chasing-the-uninterested-male-lead story. If that's what people are looking for, they can go watch Urusei Yatsura.

UKYO

Which brings me to Ukyo. In all honesty, I feel this pairing is much more realistic than Shampoo. At least with Ukyo, there is friendship and a shared history between the two of them, and their temperaments are much better matched. However, the fact remains that Ranma doesn't see her as anything more than a friend... and not from lack of trying on her part. Ukyo has been very direct and forthright about her intentions, so it's only Ranma who's left dragging his feet. And even if his engagement with the Tendos was cancelled, Ranma hasn't shown any real interest in his love life (outside the ego-boost it provides), and in fact, has been supportive and happy for Ukyo whenever he thought she'd found someone else.

Not to mention, in the grand scheme of things, like Shampoo, they're really not compatible long-term. Maybe in a few years, once they've grown and matured... but at the moment, both are too focused on their own desires and goals—Ranma on his martial arts, Ukyo on her okonimiyaki business. Not to mention Ukyo's a little too caught up in her own desires and dreams to focus on Ranma's. Even in her visions of the future, they're very Ukyo-oriented (he travels with her, selling okonomiyaki; he raises the children while she works). And if anything, her idealized vision of their future would be shattered the moment Ranma failed to live up to them—and being the insensitive ass that he is, it wouldn't take very long.

Besides, for Ukyo, I get the impression she's more caught up in the idea of love and marriage than the actual reality. Now that's she's taken in Konatsu though and is no longer alone, I think her obsession-edge may start to lessen a bit...

And if that doesn't convince you, then there's also Takahashi-sensei's own words spoken during an interview:

Interviewer: Remember that time when Ranma got really weak? Ukyo told him she’d be willing to spend the rest of her life taking care of him. I thought that was sweet.

Takahashi: That’s the sort of thing that ruins men. And if those two got married, Ranma would definitely end up walking out on her.

Interviewer: Even though she’s so devoted to him…

Takahashi: Well, yes. Her devotion is undeniable. But think about it. She always takes the initiative. She’s not really the type that would be okay just being by someone’s side, supporting them.

Interviewer: All the Ranma girls are kind of that way.

Takahashi: Hence why it’s always such a battlefield.

KODACHI

Honestly... do I even need to go there? Can we all just agree that Kodachi is undeniably insane, frightens him, and barring magic, he would NEVER ever consider dating her, let alone making her his bride?