In one or two
Scandinavian countries (can't recall which, and don't have the data in front
of me) which have legalized and accepted homosexual marriage for 2 or 3
decades, the incidence of marriage in general has dropped precipitously.
Some 60% of first births are now to single women.

Has less to do with gay marriage ( a lot less) than the fact that women are no
longer economically dependent on men.

Women's liberation has done far more to change the "social fabric" of the
western world than has homosexuality. I doubt that most of those unwed women in
Scandinavia have decided to remain single
in order to protest gay marriage.

One thing does not always have to impact another. In the same 2-3 decades that
gay marriage has become accepted in Scandinavia, the use of microwave ovens has
quintupled. Maybe we should look at
microwaves. (just joking)