“Prudes,” they would argue, should be upheld as exemplary role models because a sexually repressive society is also a society with fewer unplanned pregnancies and fewer sexually transmitted diseases.

What they really mean is that they have a religious based objection (all this, "sinning" rubbish) against other people's private sexual activity, which they then try to legitimise/rationalise by cherry picking out any negative consequences they can get their hands on.

Other examples of such failed and back to front religious logic are:

Creationists, who start with a biblical based conclusion of a young earth and then interpret (i.e. mutilate) all the available evidence to support it.

Jehovah's Witnesses, who start with the conclusion that blood transfusions are prohibited by the Bible and then list the possible complications of the procedure to support it.