This depends on where you go. My world's cultures are actually pretty diverse. In Tray and Arandar, which are human run countries, political marriages are common only in upper-middle to noble classes. Political marraige is more important in Tray than in Arandar because Tray has a feudal government (that means a lot of fighting and peace treaties involving the wedding of beautiful maidens) while Arandar is a united monarchy. Engagement can happen as early as 13 years of age. The actual wedding usually happens a little later, although marraige might be rushed if something makes it urgent. Incest is typically considered taboo for blood-related siblings. Cousins are fair-game.
In Dracran, populated by dragons. Marriage for love is almost unheard of among the majority of the population. In fact, most dragons don't marry at all. Sexually mature males will hook up with a female until she has eggs and then she is left to rear the children alone. This isn't to say that dragons are incapable of developing romantic attachments. They are, but culturally speaking, males have more "important" things to do than raise young and life is so uncertain for them that developing that kind of bond is considered dangerous. Siblings, on the other hand, develop close relationships at an early age because for vunerable little 'uns, sticking together leads to the best chance for survival. This bond does not fade after they grow up, and siblings often stay together well into adulthood. They do not, however, typically mate, but there is no specific taboo against it.
In Ter-Felix, love is surprisingly a big deal, considering they're a race of cat people. (Cats aren't considered the most romantic creatures in the world, I know.) Political marriage is frowned upon because of how important the emotional bond between the wedded couple is. Most marriages don't happen until the couple have known each other as friends for a very long time. Dating is less common than traditional courtship and sex outside of marriage as well as all forms of incest is 100% taboo. However, polyagomous relationships are perfectly okay, as long as everyone has been officially married to each other.
The scattered Chimeran race is a melting pot about everything. There are hundreds of nomadic tribes that all have wildly different traditions. Fuirmoki are an extremely localized people living in forests in Tray and near the border of Dracran. They live in small villages, have no clear upper class, and typically marry for love. Incest is usually illegal, but the laws are determined village by village, allowing for variation. Then there are basilisks, who have a similar culture except they live near water and underground. They're more interesting in the way they raise children, being amphibians and all.
So, while we're on the topic of love and marriage, how does your world generally regard LGBT+ groups including homosexuality, polyagamous relationships, asexuality, and gender dysphoria?
