STDs are in general difficult to catch, even with sexual intercourse. But this is not an excuse to have sexual relationships without condom.
Sexual fluids on the skin would never lead you to catch any STD, but if those fluids touched your mouth, genitals or anus, there would be risk of chlamydia, gonorrhea or syphilis. Also, if one of the partners is affected by herpes, there is risk of transmission by touching the lesions herpes produces on skin (visible lesions).