Janeu (Sacred Thread) is worn both by bachelors as well as married men.

Three-thread Janeu is worn by bachelors (ब्रह्मचारी) and Six thread (3 for Husband +3 for Wife = 6) is worn by married men.

(Knowledge shared and enhanced by Shiv Dayal and Ram Krishna – my friends from Vrindavan)

For more details, one can read Manusmriti.