Here's my theory. The French, especially the Provencal troubadors, led a revolution in world culture a thousand years ago in elevating to the highest cultural prestige male-female romance. This started out as largely an idealization of heterosexual romance between bachelors and women who had to marry somebody else for dynastic reasons (e.g., Lancelot and Guinevere), but soon evolved into an overwhelming cultural endorsement of the love marriage, which is perhaps the most distinctive feature of the Western Civilization of which the French have always been in the forefront. It's not a coincidence that Paris has always been the dream capital of the feminine imagination.
In contrast, in the preceding Greek and (to a lesser extent) Roman ancient world, marriage was more functional and less emotion-laden, while romance was culturally constructed to be mostly pederastic homosexuality. This is not to say that husbands and wives weren't often deeply in love in the Ancient World, just that the the most prestigious high culture tended to endorse homosexual pederasty over heterosexuality as the most favored medium of romance.
See Plato's Symposium for some jaw-dropping stuff on what erotic love meant to the Dead White European Males in classical Athens: basically, bedding boys. Their view: A strong, aristocratic man should not waste his deepest feelings on some mere woman when he could be mentoring a beardless youth in exchange for sex.
We are constantly told that homosexuality has always been oppressed and forced into a life in the shadows (as Sen. Graham might say on a different topic). Except, that's not true. In the institutional memory of both Judaism and Christianity (and perhaps Islam), which goes back to Persian and Greco-Roman times, homosexuality was what strong men did to weaker males with the full approval of society.
So, it's not terribly surprising that the French should feel more deeply than the rest of us about defending from the current fashion for same sex marriage their greatest contribution to humanity: the celebration at the highest levels of art and of cultural prestige of vive la différence.