As a parent I would not condone their behavior. I would not allow it with my knowledge. But OTOH if their urge to merge as some call it is so strong then let them marry. In Tennessee a 16 year old is considered a medical adult. They can even obtain an abortion without parents consent or knowledge.
You and a dozen others who cite instances of marriage at 13 and 21 or whatever miss the point: those instances involved marriage. Especially when the grandparents and aunts and others being mentioned here got married, marriage was for keeps.
So, the young man and young woman were making a commitment with legal, financial, social consequences. They made the commitment publicly. If one was under the legal age for marriage, a judge had to sign off on it. That meant he could interrogate the couple, assess maturity etc.
This situation involves parents letting the young man get away with total lack of accountability—get’s his buzz and has no accountability. Parents who let their daughter be used—and that’s what it is, she is being used and nothing else,
should be hung (metaphorically, of course).
There is no comparison between this and the situations people are bringing up in the comments.
Do people really not see the difference?
The key distinction here is between marriage and no-commitment sex. If a 20-something guy with a job was marriage-serious about a 16-year old daughter of mine, I would be OK with it.