I think the reasons listed at Casual Elegance sums it all up quite nicely, but here is some of my own notes to fill out the corners.

I'll just say it flat out...Ryoga and Akane would be a disaster! So would Ranma and Ukyo! Ranma and Akane deserve each other! When you think about it, they're so similar it hurts! Both are dedicated, courageous, proud, temperamental, rude but basically nice and good-natured, stubborn as mules, terrible at handling the opposite sex and can't really do anything else than martial arts. Yes, yes...I know that some of that goes for many of the other characters as well...but wake up! I don't think I have to say that they couldn't possibly be happy with anyone else. Furthermore,Takahashi-san says they're destined to be together, and who can argue with their creator

I feel that Ryoga would need someone to take a little care of him, and Akane is incompetent on that field, folks, ya can't deny it. Does she know first-aid? No, just think of the horrible "bandage" she put on Pantyhose Taro! Can she sow? Her embroidered cat looked more like a jellyfish, okay? Can she swim in case Ryoga was drowning? Not even with a life-preserver! Can she cook? Good gods, no! Her cooking is just plain lethal! If you can even call it "cooking", that is, and I'm not sure if Ryoga can cook, as I've only ever seen him cook instant noodles. And though Ranma's comments about her food are mean (but true), he's better suited to eat her food, because he's honest about how it tastes, unlike Ryoga, who just suffers through it all, eating vile stuff just because it's made by Akane, and unless you're honest, Ryoga, her cooking can never improve. It's odd that she doesn't try to improve it, though, after all she's tasted it herself and knows how bad it is. She needs a kick in the butt about it; provided by Ranma. Ryoga doesn't have the guts to do it.

Okay, so you don't have to be a housewife to take care of a man, but let's face it folks, you gotta have some basic skills, and at times it seems like all Akane can do is throw violent fits and bap you on the head. Ryoga has much less self esteem, is more easily hurt and is more melodramatic than Ranma, so if she got mad at him, the fights and insults wouldn't just bounce off of him like they do with Ranma; no, he'd probably be deeply hurt and run away, and you know how far away he can end up. Then of course you might say that Ryoga probably wouldn't give her a reason to get mad, after all he's so sweet and so on, but get real...no relationships are completely free of bickering, and we all know how short Akane's temper is. Personally, I think this is mainly due to Ranma, but she had to be somewhat inclined to have a short temper already, or it wouldn't be as bad as this. Anyway-- as you can see below, it was Ukyo who took care of Ryoga after he'd been hurt by the Hiryu Shoten Ha, leaving Akane to patch up Ranma. 

    

But even though Ukyo would like to feel feminine, I'm kinda weirded out by that complete change of personality she has in her fantasies, like I mentioned; she's the "perfect", obedient wife, thinking mostly of her husband's needs-- and that's just not Ukyo, she has way more backbone than that, so it does not seem healthy for her to be with Ranma, if she would really act like that; I mean; it seems like she loses herself! She even gave up okonomiyaki for him once, but luckily, she came to her senses. Be yourself, Ukyo-san. And also I feel that Ranma and Ukyo would have much more to gain by being friends; at least Ranma seemed to be happy when he met her again, he doesn't really have many friends (only fiancées or rivals  ^_^;) and I think he'd like having Ukyo, whom he grew up with, to talk with every once in a while without getting glomped or tricked or poisoned or seduced. 

Ranma also seems to stick up for Ryoga when it counts, as does Ryoga for him, and Ranma has admitted that they're friends; Ryoga and Ranma have mutual respect for each other, so Ranma would probably see Ukyo and Ryoga as a pretty good match; his two friends together. I think Ranma would like to make peace with Ryoga, so they could be real friends. I know he enjoys training with Ryoga. Akane and Ukyo seem to get along pretty well too, and Ukyo's methods of trying to break up Ranma and Akane have been much nicer than say, Shampoo and Kodachi's. She's been quite civil to Akane, actually, and if the whole love quadrangle was finally untangled, they would probably make good friends as well; they at least have in common that they're both pretty tomboyish, as opposed to the accented feminity of Shampoo and Kodachi, and as I have understood it, I believe Ukyo has also lost her mother, as Akane has.

Rumiko Takahashi does have a tendency to pair Ukyo and Ryoga up, like in the three-legged obstacle race and in the Tunnel of Lost Love, and though there was some confusion in both incedents, and a lot of partner swapping (Ryoga even had Ranma for a partner once in the obstacle race), both times Ryoga and Ukyo ended up together and made it through together. They made it through the Tunnel of Lost Love without being broken up by the ghosts, and they actually won the obstacle race! Too bad Ryoga was too busy brooding to enjoy their vacation in Atami. Ukyo seemed like she had a good time, though. Ah well...that's what my fic is for. ;)

As for Ukyo's cooking, Ryoga would never have to suffer through anything; she's a great cook! And Ryoga is pretty much of a (hopeless) romantic, and would probably give Ukyo the romantic attention she needs. He's the kind of guy who buys you flowers, candy and little gifts and who isn't afraid of declaring his love for you. Okay, so he is petrified of saying it to Akane, but my theory is that he doesn't say it because he is unsure of his love, not only because he is shy. Of course he is unsure-- unless he wouldn't turn on a dime everytime he saw another chance at love! I think Ryoga's "love" for Akane is pretty unsubstantial; he's just lonely and she's one of the few people who are nice to him. But the point is that Ryoga is romantic, and that's what Ukyo needs. 

And besides, I think Ranma and Akane are too deeply involved in their fiery relationship and fighting each other too much to settle down with any others. With Ryoga and Ukyo we see some of that same fire (because she is annoyed when he sulks, is destructive, clueless or gets lost, and he is annoyed when she is bossy, insultive, too businesslike or violent) but not with Ryoga and Akane or Ukyo and Ranma. That'd be just boring: sure, it'd be nice to have someone who wouldn't fight with you all the time, but shouldn't there be some conflicts? I feel that as Ukyo subdues to Ranma, so does Ryoga to Akane, moreover he's worse; he almost worships her, it seems. Bottom line, Ryoga and Ukyo turn into something other than their true selves when matched with Akane or Ranma, they sort of become the weak part of the relationship, instead of two equal parts like Ryoga and Ukyo would be if they were together. They seem comfortable around each other, and as their scheming partnership descends into friendship, hope springs for something more. 

Plus, their names match so much better, too, with the "y" and "o" in both "Ryoga" and "Ukyo" and the who "a's" in "Ranma" and "Akane". Perhaps that's a lame reason, but there you are. I have seen some people on the internet argue that Ryoga and Ukyo are nothing alike, but that's far from true. Ryoga and Ukyo are both romantics, are generally polite and sweet and both have pretty fragile hearts, much more so than Ranma and Akane, also they're both pretty lonely and know what it's like to have to take care of yourself; Ryoga on his lonesome wanderings and Ukyo running her shop alone, and the fact that Ukyo's left home and Ryoga can't find his parents and they can't find him. So they have no family life and only the semi-relationships with Akane and Ranma to latch on to, and each other to depend on for help. Everyone else in the manga doesn't seem half as lonely. And another lame reason: spatula + umbrella + ribbon + bandanna = they look good together, and we've seen them fight well together. Also, Ukyo has made an attempt at helping Ryoga get some backbone, and build up his nerve and confidence with some very... persuasive methods, as seen below.