I think we ask the same questions those do, who have lost loved ones in a war, natural catastrophy, etc.: Why does God allow evil?
Allowing something, not taking action to stop it, is not the same as willing it or causing it. The problem in the world is oftentimes the free will given to humans... The free will that allows us to choose to love God is also the free will that makes it possible to do evil. God respects this free will and its consequences and normally we live with those results, whether caused by ourselves or others. Sometimes God chooses to intervene by performing a miracle, but often not.
I often see our secondary infertility as a natural thing more than as something supernatural, although it certainly may be both at the same time. It is also a lesson, just the way everything in life can be seen as a lesson, a chance to choose God's will over my own. I think my body may have been affected by pollution, by bad nutrition growing up, by traces of bcp's in the water supply, etc. The list is rather endless... So far, at least, God has chosen to allow this cross and has not given us a miracle. Meanwhile, we try to accept His will and work towards the healing of our bodies.
Looking at my friends I have seen an interesting phenomenon: We often seem to be taught in the very things that mean the most to us. I have lots of friends who struggle with motherhood.... and are able to have children with ease. I have seen those who don't want the gift of life and are shown its value by losing a child. As for myself... I have always loved children and asked for nothing more than to be a mom. I have also been taught "right where it hurts the most." While I cannot say whether God willed this or allowed this (or if both can be the same thing), I can see some of the value of this lesson taught. It has taken me a long time to truly say "Your will be done" but I am there now, most days... and that is a big thing for me, to give away, freely, the very thing I have always dreamed of. (Many kids.)
I do struggle with bitterness at times. You know, why does God allow this lesson for me but now for so many others? Yet, I do see that most people have their crosses, and I would not want to trade with them, either.
We will not do much outside of testing and vitamins, etc. (No male test outside of the marital bed, that sort of thing.) I am not sure about Clomid. I can see the possible problems with multiples. Other than that, I like what I once heard said: If you have broken pipes, you call a plumber. You don't just sit in front of the sink and pray. So I don't have a problem with medication, really, depending on how it is used. We will not do anything where the child is conceived outside of the marital act.
Whatever the whys behind all this are, I do see value in this also. I join my sufferings with those of Christ's and ask Him to use them for the good of others.