Can you lay out how everything else follows, or how consent is a recent innovation? Neither is not obvious to me.
The line of reasoning I've heard is that intersex marriages produce families, which societies value.
Same-sex marriages do not produce children, as a simple result of biology. I don't deny that a same-sex marriage can form an effective family through adoption or surrogates, nor that an intersex marriage can fail to produce children, but the default result is children in intersex marriages and no children in same-sex marriages.
If society wants to encourage children and families (for obvious reasons), one simple route is to encourage intersex marriage. Same-sex marriage is, in this supposed society, 'discriminated against' because it doesn't produce the same result as intersex marriage, in the same way that you can't marry yourself, or 'donate' your income to buy a new car for yourself instead of to a charity and expect a tax break, or spend your retirement savings tax-free before retirement.
I also don't deny that this simple route of encouraging the raising of children by encouraging intersex marriage isn't perfect, and the imperfections cause harm. But it's kind of baked into our culture and legal system. Suggestions that simply extend the encouragement to same-sex marriages to eliminate the harm without fixing the encouragement are also incoherent, failing to see the reasoning for the fence. I wish you good luck if you want to take away the benefits from childless intersex marriages, or not extend them to childless same-sex marriages. And a lot of the noise around the issue is based on emotional, theological or incoherent reasoning, which makes sensible communication difficult.