I recall either some state court or the Supreme Court ruling that the minimum marriage age for boys & girls be equalized. In that ruling, the marriage age for a boy was reduced to 18, but if the spirit of that law is kept, a common minumum of 21 would be welcome
The ban on polygamy as well as marriage to relatives is welcome, & should be implemented across the board
A minimum age of 21 for marriage may make sense so that women have a chance to complete college before their families react to their sexual activity by putting pressure on them to get married. However, keeping the minimum age at 21 for men or raising it for women means that those who can't afford college education still can't get married when they are already having children.
If two consenting adults who are relatives want to get married, I don't think my distaste for it should outweigh their freedom. However, a more prevalent situation is that an older relative courts/exploits a younger relative in the same household or extended family, and this power imbalance turns into a cousin-marriage or uncle-niece marriage. The ban on marriage to relatives could liberate children who are at risk of being tied for life to predatory family members.