The Church does say masturbation is just as immoral as gay sex, or non-vaginal sex (even with a straight married couple), for this very reason.
I'll let TJ or somebody parse out the good points and bad points of this assertion.
I should have included condoms and similar birth control too.
The theology boils down to: all sexual acts must be "open to life."
I took Catholic theology for 4 years, and other religious studies classes in college.
What concerned me is that, given the issue that the proposed law is obviously in reference to, it seemed like you were implying that "this very reason" is the same reason given for the Church's opposition to abortion, which it isn't. Nobody seriously argues that masturbation or gay sex or using a condom is homicidal.