It is thought that the split between female factor and male factor infertility could be 50-50.
A partner’s health unequivocally matters.
Oxidative stress is the largest concern for sperm health. Causes of oxidative stress include aging, varicocele, testicular torsion, and cryptorchidism. Controllable sources of oxidative stress are alcohol consumption, environmental toxins, cigarette smoking, and exposure to radiation, and insulin resistance. (Note that the level of damage we are talking about with insulin resistance puts it in league with smoking - and we already know that's a problem for sperm health.)
Oxidative stress harms the fluidity of sperm plasma membrane, decreases sperm motility, decreases ability to fuse with an oocyte, as well as alters the sperm DNA integrity.
Strikingly, diabetic men (high insulin resistance) who exhibited normal semen parameters on semen analysis still had fundamentally more elevated amounts of defect in sperm mitochondrial and nuclear DNA, presumably because of elevated blood glucose levels and associated oxidative stress.
Increasing antioxidants can show measurable levels in the seminal fluid, and can secure sperm DNA. The main studied antioxidants are Vitamin C, Vitamin E, carnitine, CoQ10, beta carotenes, and glutathione.