Marriage in the plot
-Arranged marriage: Marriage arranged by family members, without mc's consent and sometimes even without his knowledge
-Forced marriage: Different from arranged marriage in the sense that it's more sudden and one-sided. For example the mc is told, "due to the position you are in/ you just acquired, you have to marry this person", or an influential person orders the mc to marry her.
-Abnormal marriage system: Like "Koi to Uso" where the government introduces you to a partner at the age of 16 and you are forced to marry her/him, or "Arcane Transmogrification" and its prequel book (both from this site) where marriage always occur between 3 women and 1 man. Another example would be "Loner Harem Meister" where a man can legally marry up to ten women, but the government punishes with taxes and other things those men who stay single.