How far into vaginal intercourse does it take to actually “exchange bodily fluids” and risk getting an STD? What if it “slips in” for not more than five seconds with possible pre-cum and no ejaculation?

Exchange of bodily fluids and infection work both ways in vaginal intercourse. Sexually transmitted infections are carried in ejaculate, pre-ejaculate, and in vaginal secretions. So any contact between the penis and vagina is a risk for transmission of infection — for both partners.

It’s also important to remember that intercourse is not necessary to spread many infections. Herpes, HPV, scabies, and pubic lice are all spread through skin-to-skin contact.