I take it from your screen name you've got some scientific perspective on this question. Is it correct to say that even with correct and consistent use the device itself will have a certain level of failure?
If the answer is affirmative, then that will be the insurmountable hurdle for any examination from the ethical perspective of the Church. There will be, in fact, no "lesser evil" to consider. Only the greater evil of transmission of a potentially fatal disease delayed. (Unless you have the statistical bad luck to have your first use be a failure. Then there will be no delay.)
I take it from your screen name you've got some scientific perspective on this question. Is it correct to say that even with correct and consistent use the device itself will have a certain level of failure?
The answer to your question is yes; no barrier is 100% foolproof, and people won't always use it correctly.
I'm on the medical perspective end of this, and lacking on the theology end. I don't really understand how any degree of failure makes an insurmountable hurdle - but I welcome the education.