Welcome to the STD forum. Responding first to the title you chose, before reading the question: it is not possible to catch an STD through clothing. If a bare penis doesn't enter a vagina, rectum, or mouth, generally there is little or no STD risk.
Now I have read the question. It is probable that nobody ever caught an STD through clothing, despite moisture from a partner's genital secretions. There might be a small, theoretical risk for transmission of the skin-to-skin STDs, like herpes, HPV, and syphilis -- but too low to worry about. You don't need testing and can continue unprotected sex with your regular partner(s) without risk.
Here is a thread that discusses the biology of STD transmission in detail, explaining why exposures like this carry little or no risk. You should find it reassuring. Start reading with the folllow-up comments that start December 14: http://www.medhelp.org/posts/show/1119533
i had sex w a classmate. we used a condom. the area around my penis and my underwear were soaked with her vaginal fluid. could i be @ risk because of her vf on my underwear in the location of my penis? like my urethra?
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