Perhaps some writers are out to prove that this is not a kiddie fandom, by focusing on the most "adult" subjects they can think of?

That could very well be. I'm also convinced that many ship writers are filling the vacuum left by the canon, which has very little romance.

Also, consider the secondary meaning of "adult" in popular usage - it refers to "sexual content." When we talk of "adult entertainment" most people don't mean a night at the opera. I wonder if, subconsciously, some writers want to include sex and romance so as to seem more sophisticated and mature? I doubt this is done with intention but it does bring up the use of "adult" in our lexicon.