Direct spread of other STDs via direct spread is rare and very few cases have been documented. This does not mean that they do not occur. As such common sense should always prevail when engaging in any form of sexual encounter. Any obvious sign of infection like syphilitic chancres, cold sores, genital ulcers, vaginal/penile discharge etc. etc. should serve as a red flag.
Direct spread can occur from one person to another without necessarily having engaged in any penetrative sexual intercourse. This can happen in cases where one infected partner touches their genitals before touching their partner’s genitals like in mutual masturbation. When it does happen they can pick pathogens and transfer them to the uninfected partner.