Thank you for writing.
Pre-ejaculate itself does not contain any sperm. Sperm may be inside the urethra if a man ejaculated recently and did not urinate since. If he urinated since he last ejaculated, there is no sperm at all. Pregnancy is very uncommon from pre-ejaculate and the urethra would have to contain sperm at the same time. A woman would also have to be ovulating and engage in unprotected intercourse with ejaculation. Sperm cannot penetrate a layer of cloth.