I am under impression that the SM aren't sterile, their drive is just suppressed to the point of non-existence.
Also, since the gene-seed is what transforms a man into a space-marine, a child of a space marine and a normal human women would be like the child of that woman and the man he used to be.. I think
Just saying. From everything I read, I don't think the SM genes can be transferred normally, nor is there anything implying sterility. Hypno-conditioning and chemo therapy mention nothing about it in any source I've seen.
However, since those that become SM's are chosen for already being the best human stock, any hypothetical child of a SM would be exceptional anyway. Just not a Space Marine