Exactly Lighter. Correlation does not imply causation. Instead, correlated phenomena may be linked to some other factor (in this case level of education perhaps?) that could be considered as a causative factor.

Nate Silver acknowledges this, and closes with:

"At the very least, I would be surprised if there were any statistical evidence that interpreting the right of marriage to apply to same-sex couples would be injurious to heterosexual couples in any material way."