The answer to me isn't obvious -- and by that I mean that there's so much wrong with the situation that it's hard to know where to begin.
OK, let's start with the imbalance of power. A 15-year-old can't sign a check, can't have a bank account, can't get a job. Good grief, you can't even get a driver license in California at 15!
Then there's the imablance in life experience. A 25-year-old guy has lived nearly twice as long as the 15-year-old. The guy could have been in the Army or the Marines for seven years already (or three years into his Ph.D studies), and she's still in high school! What do they have in common? How can they have anything like a normal dating relationship?
We once went to a seminar titled "When an Adolescent Invades Your Home," and the instructor pointed out that the lights aren't really on for a significant part of the time until they get to be ten or twelve years old. So let's say it's 10. The 15 year old has had 5 years of life with the lights on, but the 25-year-old has had 20 -- that's a 4:1 ratio.
Suppose the guy was 40 and the girl was 15 -- i.e., he's old enough to be her father. What would your objections be to such a "dating" relationship? The total imbalance in life experience, power, perspective, etc., right? What's different if he's 25?
I also would have to wonder what a 25-year-old man wants from a dating relationship with 15-year-old girl (besides the obvious I mean). And what kind of 35-year-old he'll be. Will he still be interested in the same topics as teenaged girls? I mean, is his social, psychological, spiritual, intellectual growth simply delayed, or has it come to a halt?
Now I recognize that I'm speaking from a particular cultural perspective, and that Mary the mother of Jesus was likely a teen-ager whereas Joseph her husband was likely much older. But in those days high school hadn't been invented, women didn't go to college, etc. Come to think of it, I don't think dating had been invented. And in those days the "He's 25, she's 15" was just fine. So at the risk of coming off like a western chauvanist, I'll say that if you want your 15-year-old daughter to live in a world like that, it's not incongruous to say it's fine for her to date a 25-year-old man.
But I want each of my daughters to have the opportunity to pursue her academic career, find and develop her vocation, and have a husband that will be a good match for her. Being in high school and having a 25-year-old boyfriend is not in line with that trajectory and is frankly rather creepy.