Do very low free testosterone levels (below 100) usually go along with infertility in a male?

Depends . . .: It's possible to have issues w/enough lh or leydig cells such that testosterone is low yet fsh ; sertoli cells are fine so no fertility issues. However, taking testosterone in any shape or form will essentially render you infertile as it will shut down your fsh and thus sperm production. If you need to boost your t and maintain fertility, consider off-label hcg, Clomiphene or tamoxifen. Good luck!