Can a precum pass through brief and jeans?

No. If you are to be intimate with your partner you should really understand how a woman becomes pregnant, and how contraception can help prevent unwanted pregnancies and STDs. https://www.plannedparenthood.org/learn/pregnancy

The moisture may pass through, but sperm are fragile & die in minutes when outside of the moist protective environment of the vagina. The chances of them being functional enough to trigger pregnancy approaches zero.

Sperm will not pass. While you are not at risk of pregnancy from this episode, your behavior is putting you at risk of pregnancy and STDS. Visit this site for guidance on safe sex: https://www.hopkinsmedicine.org/health/wellness-and-prevention/safer-sex-guidelines Get HPV vaccine Use contraceptive all the time, every time. Wish you good health!