Finally weighing in on this topic. I have sort of skimmed the thread since it was opened, but I didn't read everything, and I don't remember everything I did read - so I apologize if it seems like I'm repeating what someone else said instead of quoting them to agree. It's not intentional.
I think the marriage was a mistake because it feels like it was done out of convenience rather than as a conscious creative choice - and it wasn't the kind of spontaneous, go-with-the-flow plot choice that turns out to be even more awesome than whatever you may have been originally planning. Quite the opposite, I'm afraid. Even if TPTB were planning to have Danny and Lindsay get married eventually, shoehorning it into season five to accommodate Anna's pregnancy was a mistake. (They didn't
have to get married to accommodate the pregnancy* since people have children without being married all of the time, but you can't convince me that the decision to marry them wasn't dictated by the need to have The Messer Family be 'official' and prevent Lucy from being, much as I hate the term, a bastard child.)
The marriage has done neither character any favors. It hasn't guaranteed either one of them any measurable amount of development together (the potential for development during Danny's recovery at the start of season 6 was largely wasted since they rushed through that storyline), and it seems to have removed even the
possibility of separate development - will either character have anything that doesn't involve the other at this point? Doubtful.
Yes, marriage is a commitment and a union, and I'm not disputing that - but on a show that lasts 42 minutes (most of which is taken up by solving the Crime of the Week), having your Married Couple spend a great deal of their scenes together and with no focus on outside relationships (work, family, friendship, etc) is very limiting for what you can do, creatively, with both characters. Danny and Lindsay have been utterly bland for pretty much the whole season, and almost every spark of potential interest gets swallowed up by the need to always make sure one doesn't go anywhere without the other (literally or figuratively) - which is a shame when they could be making use of
every unique and interesting relationship on the show.
So I don't think it was out of left field, not in the sense that they were probably going to get married eventually anyway, but I think it was a mistake to make it happen within the span of one season with so little logical build-up. (And quite frankly, any storyline that requires you to retcon events from a previous season to push the plot along is a mistake in my book. Danny sleeping with Rikki was suddenly a mistake that was all about cheating on Lindsay and nothing to do with, ya know, falling together in shared grief because you're trying to comfort yourself and each other over the loss of That Dead Kid during a time when D/L was ambiguous at best? Um, no. :wtf
* The decision to write in Anna's pregnancy instead of writing around it was another mistake if you ask me - it may have been more convenient for everyone involved, but it wasn't the best choice creatively, certainly not considering the way they went about it. Even if it does provide an easy excuse should Anna and her husband expand their family with another child - which is their right, and I'll be happy for them if they do, but there's no denying that it's an easy way out to just say, 'Oh, they're adding another bambino to The Messer Family' if Anna needs time away from the show.