It's kind of funny. If this was a 20 year old guy and an 18 year old girl, no one would be asking this, and they probably wouldn't meet the parents for months, but because there is an age gap, suddenly he has to seek the parents' permission before even considering dating the girl.
I bet if she wanted to go out and sleep around at the weekend, no one would ask about her parents' input then.
There's just this weird resistance when it comes to older guys being able to sleep with younger women. Western women have these guys really well trained. They'd probably be more angry at and older guy marrying a girl than they would a young guy dating her and treating her badly.