Children were the reason marriage was invented; The anticipation of children being fatherless or motherless was something a tribe or community could not tolerate. They watched and saw enough miscreants created this way that the institution of marriage was invented as a remedy for the fatherless/motherless home.
Enter Obergefell which fundamentally reversed that contractual enjoyment children used to have.
As to mdk's assertion that "if something doesn't exist then it cannot be an implicit part of a contract", let me say this, children not being born yet, but anticipated by the society that sets up marriage parameters is the same as profits not yet in existence from a contract signed between budding business partners. Those profits anticipated are in fact part of that contract. So are children anticipated to come statistically from marriage.
It is that very anticipation and not wanting a tribe full of miscreants that led states to incentivize marriage with benefits in the first place. Now they are "legally" (not) required to incentivize THE EXACT OPPOSITE OF THE ORIGINAL INTENT OF THE CREATION OF THE MARRIAGE CONTRACT.
Thanks to five unelected lawyers in DC in 2015.