Could just choose to marry neither. That would solve the problem of favouritism. In Denmark, at least, it's not strictly necessary, as in the eyes of the law, people who've been living together as a couple long enough are in most regards equal to those who are married. Dunno how it is in Japan.
Copied from Wikipedia:
"In Japan, the Japanese Penal Code sets a minimum age of consent of 13 regardless of gender or sexual orientation. However, the Children Welfare Act chapter 34 forbids any act of 淫行 (which can mean fornication, harlotry or obscenity) with children (here defined as anyone under 18 years of age) Specifically this is number 34(1)vi.
Municipalities and prefectures have their own particular laws, such as Tokyo's Youth Protection Law (Japanese: æ±äº¬éƒ½é’å°‘å¹´ã®å¥å…¨ãªè‚²æˆã«é–¢ã™ã‚‹æ¡ä¾‹) which prohibits sexual activity with youths who are under 18 years old in most circumstances though some add exemptions for sex in the context of a sincere romantic relationship (typically determined by parental approval)."