remember, a healthy 24 yr old male produces 600 million viable sperm per ejaculation.. anything that will effect that # including age will lower your chances of getting prego..
Also remember, it only takes 1 sperm to impregnate a woman and finally, a vasectomy is covered 100% by most insurance companies (that's right)
oh and one more thing..
up to 40% of all births.. the men raising the child is not the biological father.. so the statistics say..