I see, I see, marrying your surrogate daughter is much more realistic than two adults coming together, who have been attracted to each other for years.
Statements like this really make my day.
Hahaha, if you've ever been in a real world adult relationship it's usually the more complicated situation that plays out rather than the situation you wish for. In rare exceptions there are happy endings, so if you can relate to that more, all the happiness to you.
But since you have to ask...
Daikichi never makes it really clear why he wanted to take care of Rin in the series. There's a number of times when he tries to form coherent reasons (felt sorry for her, wanted to be a better man, felt like he could provide for her something no one else could) but most of these reasons would not justify the level of passion he puts into raising Rin. Most people, I think, would never make that kind of self-sacrifice for someone they aren't related to, unless they are in a situation where it makes sense for them to adopt a child.
Daikichi also isn't that dumb or crazy; he's got responsibilities as a manager at his workplace and has enough social skills to interact normally with his co-workers and other parents. And as a single male, and not being gay (he's attracted to Nitani-san, obviously), he must surely have normal sexual needs.
Clearly the solution that the author imposes at the end of the series isn't the best one nor a satisfying one. I agree with that completely. But what she's trying to find in that awkward mess between Daikichi and Rin is advocating a type of relationship that exceeds typical ones because of the emotional bond between the two. I'd say most marriages hardly ever have that level of dependency and willingness to sacrifice. Rin is always trying to figure out what Daikichi means to her, because he's always more than just a father figure. That's why from the beginning of the series Rin never calls him "father" but by name. As childish as that may be, Daikichi never fulfills a defined family role but something more in Rin's mind (and heart). It's only towards the end that Daikichi fully understands Rin's needs, which is a dependable man in her life. He's of course conflicted (portrayed by his inner dialogue spoken out loud), but he also acknowledges his own needs (a caring female figure) in resigning to the situation.
*edit: some of this post echoes Pocru's words above. had to put it in my own words, maybe it'll help?