For our purposes here, when we say ‘fucking’ we mean penetrative sex where a penis is being put inside someone anywhere except a mouth. In general, this is considered ‘high risk’ for STIs because a lot of bodily fluid is being passed from one person to another.
One way of making fucking safer is to use a condom, which provides very good protection against most STIs. ‘Barebacking’ or ‘bb’ is a common term for fucking without a condom, which is considered high risk. Using lubricant and taking PrEP are ways of reducing the risk from barebacking. Some people are willing to accept the risk but try to lessen it by pulling their penis out of their partner before cumming. This does reduce the risk, but not to zero.