Begging Technique

From the Wiki:
"First-cousin marriage is allowed in Japan, though
the incidence has declined in recent years."

That's why our frisky youngsters aren't too
concerned about their parents' reaction.

It's still legal in most of the world, though much less common now. A bunch of countries also still allow avunculate marriage; that is, marriage between aunt/nephew or uncle/niece.
Interestingly, in Canada, you cannot legally marry your stepsibling, despite having no blood relations, but you CAN legally marry your blood-related aunt/uncle or niece/nephew.