Does using leather ball stretcher cause infertility?

Yes, it very well could. Sperm are manufactured by the body in the srotum as they are because they have to be at a certain temperature to be live and motile. If they are too warm or too cool (as a "ball stretcher" would cause them to be) they will die or not function properly. Most of the body works the way it does for a purpose and altering that can lead to the body not functioning properly. "Stretching" can also damage the vas deferens which are the ducts that transport sperm in anticipation of ejaculation. Testicular torsion is the most serious potential consequence of scrotal stretching. If the spermatic cord and blood vessels are sufficiently compressed, circulation to one or both testicles may be compromised. This will cause swelling, bleeding and pain and is considered a surgical emergency. A surgeon may need to fix the attachment of the affected testicle as well as check for effects to the other testicle. If medical treatment is not sought immediately, the testicle may become gangrenous and require removal. f rings or weights are left in place for a significant period of time, as they must be in order to achieve desired results, one unpleasant consequence may be inflammation secondary to a skin infection owing to poor hygiene, with the scrotal skin going unwashed or not being "aired out" for considerable periods. Anaerobic bacteria may thrive in the absence of oxygen, leading to the formation of abscesses, foul-smelling pus, and even tissue destruction, especially if cuts are present. As all males know, bruising and swelling of the testicles is very common, and scrotal stretching carries risks of these effects because of the placement of devices against sensitive tissues. If, for example, a strap is accidentally pulled against rather than above one or both testicles, swelling may occur; damaged veins over time could cause a varicocele, while fluid buildup in the aggravated scrotum can cause a hydrocele, both of which are painful. Most testicular bruising and swelling resolves within a few days. If stretching devices remain in place, however, pain and swelling may persist for longer.

I would think doing that would be dangerous if its not something your dr says you need. I have never heard of anyone voluntarily using such an instrument of torture. I can see where it would temporarily cause problems with fertility. The purpose of the scotum moving away and to your body is to keep the sperm at a temperature they can tolerate. Keeping them away from your body may kill some sperm.