What causes a mans cum to be stringy?

Protein. Semen is a product of several things. There are secretions from the testicles themselves, secretions from the prostate and from the seminal vessicles and the cowper's glands along with mucous from the urethra. All of these have amounts of carbohydrates but the majority is protein. The protein nature makes the seminal fluid viscid, thick and stringy. With time it liquifies & stringy.
Normal semen. Semen consists of milky clear fluid from the prostate and seminal vesicles (90%). The clear stringy 'jelly' is the actual sperm. After ejaculation when these components are mixed together, they liquify after 15-20min. That's how the sperm make it past the cervix and up the uterus for fertilization.