One of the big specific changes was the fact marriage got redefined with the end of coverture.

"Coverture" isn't even in Firefox's spell-check, which I suppose points to how obscure it is these days. It used to be part of what marriage was: It was the fact that a married couple was a single legal person for many purposes, that person being the husband. Wives couldn't own property in their own names, couldn't have bank accounts in their own names, and, to a large extent, couldn't exist in their own names, but had to rely on their husband to function as an adult.

https://en.wikipedia.org/wiki/Coverture

> Coverture (sometimes spelled couverture) was a legal doctrine whereby, upon marriage, a woman's legal rights and obligations were subsumed by those of her husband, in accordance with the wife's legal status of feme covert. An unmarried woman, a feme sole, had the right to own property and make contracts in her own name. Coverture arises from the legal fiction that a husband and wife are one person.

[snip]

> After the rise of the women's rights movement in the mid-19th century, coverture came under increasing criticism as oppressive towards women, hindering them from exercising ordinary property rights and entering professions. Coverture was first substantially modified by late 19th century Married Women's Property Acts passed in various common-law legal jurisdictions, and was weakened and eventually eliminated by subsequent reforms. Certain aspects of coverture (mainly concerned with preventing a wife from unilaterally incurring major financial obligations for which her husband would be liable) survived as late as the 1960s in some states of the United States.

Quoting Blackstone:

> By marriage, the husband and wife are one person in law: that is, the very being or legal existence of the woman is suspended during the marriage, or at least is incorporated and consolidated into that of the husband: under whose wing, protection, and cover, she performs every thing; and is therefore called in our law-French a feme-covert; is said to be covert-baron, or under the protection and influence of her husband, her baron, or lord; and her condition during her marriage is called her coverture. Upon this principle, of a union of person in husband and wife, depend almost all the legal rights, duties, and disabilities, that either of them acquire by the marriage. I speak not at present of the rights of property, but of such as are merely personal. For this reason, a man cannot grant any thing to his wife, or enter into covenant with her: for the grant would be to suppose her separate existence; and to covenant with her, would be only to covenant with himself: and therefore it is also generally true, that all compacts made between husband and wife, when single, are voided by the intermarriage.

Same-sex marriage wasn't a redefinition of marriage: Same-sex marriages are exactly like all other modern marriages, by which I mean post-coverture marriages, in terms of the rights and abilities of the married persons and how they relate to each other legally. However, same-sex marriage is flatly impossible in any system with coverture: There couldn't be two males in a marriage, because there could only be one legal person, and there couldn't be two females in a marriage, because there had to be a legal person. It was as if the law was asking "Which one of you is the husband?" and expecting an answer.