Why, may I ask the court, is it now illegal to be involved and/or married to a 13 year old? (And please don't just say because it's illegal. The question is "why".)
I presume you're asking why it SHOULD be illegal. (Clearly one reason it actually is is that most of society is horrified by the idea.)
That's a big topic, obviously. I suspect many professionals would argue that on average, girls would be worse off if it was legalized than if it remains illegal. I have no doubt that they are sincere in that conviction. I also think they are right, though for reasons more complicated than simple slogans. I addressed the whole thing in a series of essays that are indexed here (http://celibatepedos.blogspot.com/2017/03/index-considering-pro-legalization-pro.html).
But the reason you ask that question after discussion Poe and his wife is that you consider that relevant. One case is hardly ever persuasive. Some guy will tell you how he played Russian Roulette twenty times and nothing bad happened to him -- he found it exhilarating. But sensible people will tell you that would be very dangerous (chance of death 97.4% assuming a "fair" six-shooter). There are cases where parents forget their sleeping tots in car seats in hot parking lots and the kids die ("Fatal Distraction" by Gene Weingarten is interesting though behind a Washington Post pay wall). Quite likely they would have survived if they weren't locked into car seats. Advocates of car seats will claim that the net benefit of the seats is positive even though in a few cases it's harmful.
Though their marriage was loving, some biographers suggest they viewed one another more like a brother and sister. --Yeah. Right.
I can understand your skepticism, but surely you can't be sure you are right either? She apparently never became pregnant.
It was also a different era. We have higher expectations for girls these days -- that they can get an education and make a decision about things like marriage at 18 or older, even if it's not illegal for them to marry a little younger in some places. Even then, Wikipedia says they falsely claimed she was 21 at their wedding ceremony.
The radical Youth Rights brigade will now say that total freedom is more important than bad outcomes, and I'm reluctant to get into that argument again. I suppose 6-year-olds should also be free to play Russian roulette.