No children because of no sex

We are married for 8 years, but we have no children. We are both healthy with one exception - we hardly ever have sex. Why? I don't know. 
I brought this subject to him for quite a few times, I threaten to leave if nothing will change. I also thought to have an IVF and keep relationship as in general we get along well. But then I came to conclusion, that  it is not right. I like sex and there is definitely something wrong between us if he doesn't want me. 
Again, he doesn't cheat and doesn't have any problem with health. I am in a good shape and look after myself. But he simply doesn't want to have sex.
I'm 33 and I feel my clock ticking and I really want a child. I see families with kids outside and get upset trying to understand how on earth it is possible that a man is not interested in sex. 
What shal I do???