Yes, I know -- my husband and I are suffering with infertility. It is not our intent that procreation be separate from the conjugal act -- but it is the reality.
We have not yet done IVF; I don't know if we will. I am taking fertility drugs. And I can tell you that with doctors involved in our intimate life, it is not the "normal" way of things. Doctors track when it's time via ultrasound and bloodtests, and give us our instructions. I guarantee you that under these circumstances, it is much more like a laboratory than a conjugal act. But like most people who pursue infertility treatments -- IVF or otherwise -- we are going through this horrible, clinical process because of our strong love for each other, our strong marriage and our yearning to have children together.
Some might say that we are trying to thwart God by turning to medical help; but that is the same as saying people seeking help for cancer, or heart disease are trying to thwart Him. We're supposed to turn to Him, but also to work, and to help ourselves -- and we are doing all of the above. If the Lord doesn't want us to have biological children, there is no way that someone in a lab coat is going to be able to make it happen.
I must disagree with your assertion that seeking IVF and other illicit technologies are on par with cancer therapy or the like. That is not even close to the truth. We certainly have a right to correct a pathologic condition, within reason, but we do not have any right to correct it by using immoral means. Just because I may be sick does not mean I have recourse to every possible way to correct it if it means violating the moral law.
Also, people in lab coats, like other folks, have a free will. God will not violate anyone's free will. So, even if they want to commit an evil, God will not stop them. If they want to thwart God's will and plan for human reproduction, He will not stop them. In fact, like all sin, at some point God will turn them over completely to their sin as punishment. They will seek even greater ways to thwart God's will, and still say it is not sinful.