A MIDRASH on Canaanites and same-sex marriage? So says David Klinghoffer at Beliefnet:

Canaanites for Same-Sex Marriage
Thursday April 30, 2009
Categories: Culture War
An ancient Biblical tradition, a midrash, relates that the Canaanites wrote marriage contracts between man and man and woman and woman, and that this was one reason the land "vomited" them up in favor of the Israelites who took their place. The historicity of this isn't the point. It's the moral that matters, having to do with the social impact of being libertarian about marriage combinations

[...]

I have no wish to get into the modern controversy here, but I am interested in the midrash. As usual with such pieces, no reference is given. Is there such a midrashic text and does someone have the reference?

UPDATE: A number of replies. From Dorothy King:

I'm guessing the Midrash linked to Leviticus ... and think this is the one they mean: Sifra Acharei Mot, parashah 9:8

From Jeff Wasserstein:

I'm aware of the midrash about writing ketubot for men, and googling it (since it's been years since I saw it inside) suggest it comes from here: Hullin 92b; Yalkut Shimoni, Zekhariah, no. 578;

Vayikra Rabbah 18:13. Article from which I drew these sources is: http://www.bethelsudbury.org/uploads/9407Homosexuality.pdf

From Simon Montagu:

The reference is to the Sifra 8:8, on Leviticus 18:2

ומה היו עושים? האיש נושא לאיש והאשה לאשה, האיש נושא אשה ובתה, והאשה ניסת לשנים. לכך אמר ובחוקותיהם לא תלכו

The Hebrew can be translated as, "And what did they used to do? A man married a man and a woman a woman, a man married a woman and her daughter, and a woman married two men. Therefore it says, By their rules you shall not walk."

From Michael Pitkowsky:

This midrash is found in a number of places: Sifra, Aharei Mot, 9:6-8 (speaks of marriage but no ketubbah); Genesis Rabbah 26:5; Hullin 92a-b. None of them mention the Canaanites by name. You may find the attached article interesting, especially pages 129 ff.

The article is BETH A. BERKOWITZ, "The Limits of ‘‘Their Laws’’: Ancient Rabbinic Controversies about Jewishness (and Non-Jewishness)," Jewish Quarterly Review 99 (2009): 121-57.

Many thanks to those who replied.

UPDATE (4 May): Dorothy King has reflections here.