Digory and Polly's situation is really quite realistic: a boy and girl go through some experiences together, and then grow apart. But then they stay in contact, and are friends as they age. From the comments made in LB, we know that the children call Polly 'Aunt', and she is clearly part of the group of Friends of Narnia.

[It's also important to look at it in the context of World War 1, when many young men were killed in battle, and many young women remained single. So Polly's singleness in later life might well be due to losing her sweetheart, and the reduced number of men in that generation; friendship with other men didn't necessarily lead to marriage] 

I'm sorry that you are disappointed at the lack of romance in this book, but the children are only 12 and 11 (both pre-teen, in today's terms).

Shasta and Aravis, by contrast, are 14 and 13 (making Aravis old enough for her father to marry her off). In this case a boy and girl go through some experiences together and separately, then stay in contact, and some years later decide to get married (Lewis jokes that it is so that they can keep arguing). 

If Lewis had married off both pairs, I think there would have been some complaints that he wasn't realistic, because not all pairs of boy/girl friends end up marrying each other!