In 2019, why is it inappropriate for enlisted and officers to date or marry with consequence?

You're in the reserve component, you're allowed to date an officer as long as it's not from your work. If you don't understand why dating someone at your work would cause major issues, then it can't be explained

If you truly believe in the necessity of the division of commissioned and enlisted personnel, you cannot countenance such unions. Once that divide is bridged in the bedroom, it ceases to exist anywhere else. I know that there are commissioned officers who want to be "familiar" with subordinates. I've seen them and, in every case, they were ineffective leaders not only to their own detriment but also to the detriment of those they led. Command is a lonely space. Responsibility is a heavy load to bear. If you aren't up to the challenges, you should remain among the enlisted ranks.

Can theoretically be married, just can't date, which essentially makes it impossible to reach "married" without having an improper relationship, other than one member of an enlisted marriage becoming an officer after the marraige.
The "why" is because it is codified in Army Command Policy, and the resulting charge could be a violation of Article 92, UCMJ.
It used to be authorized, but was again prohibited 1 March 2000, update to AR 600-20, para 4-14c(2), and remains so with the latest update.

Whether it is 1819 or 2019 is irrelevant. Officers and enlisted don't play with each other. Just like lower enlisted and senior enlisted don't. There is supposed to be a separation, which allows for military order and discipline. Even when not in the direct CoC, the potential to influence a career exists.
In the way way back, I had a drinking buddy who was CID. He would stop by the unit approx three times a year, flash his badge, and take me away from the unit for an afternoon to go shoot the shit. Looking back, this was EXTREMELY wrong, but at the time it was awesome to get out of an afternoon of mowing grass or cleaning weapons. An Officer has the potential for a similar type activity, even with a different CoC. MAJ Jones comes in and talks to CO or 1SG "hey, need to borrow SSG Smith for the afternoon." CO will likely agree, and now SSG Smith is not doing the job because MAJ Jones wants to have some together time.

Its complicated, but reality is that it happens.

I suppose I could say because those in charge of believed to be true and needed but that’s not really a discussion.
So instead I would suggest you
Read the regulation understand why it’s deemed inappropriate and tell us why you think those things are no longer valid

My husband was an officer before he got out. At one time he was active and I was reserve. We had known each ither and were married while I was in college shortly after he graduated. Then he got off active and were both reserves. Most of the time we were separate units. His last unit was the same as mine but he was ops and I was maintenance so not the same chain of command. It happens quite a bit in the guard or reserves. We actually work at the same place in our civilian careers now but again, not the same department or area of expertise. Its nice to have “lunch date” every day.

Why? Simple yet effective example.
Let's say you as a SSG are married to a Captain. The two of you are in a situation where lives depend on the decisions, the impartial, unemotional decisions that must be made. A spouse that has to order their SO to 'take the hill' so to speak knowing they could die. Not to mention the sheer fact that it would be bad for moral as favoritism would always be a lurking factor. You did not earn your stripes but got them due to the rank of your spouse and so on. This degrades not just your leadership ability but the leadership ability of your Captain spouse.

It leads to a degradation of good military order and perception.

Hmmm... I remember attending the wedding (1976) of Air Force Lt. X (can't remember her name) and Airman Senior Ray Garcia. Now granted, that was Air Force, but did the rules change?

No one has mentioned different social classes of people.