Many common lubricants contain substances that slow down the unrestricted movement of sperm, impair their longevity as well as attack their DNA.
Not only the components but also the consistency of the products can contribute to this undesirable effect. (Note: lubricants are not a safe nor an effective contraceptive!)
Saliva as a lubricant is also not recommended. The enzymes it contains can denature the proteins in the sperm and lead to sperm loss.