If you look at the link below, you’ll see that the 70% figure comes from a study of repeated sex over time (with self-reported condom usage), and so can’t be applied to the figures for a single sex act. Preventing semen from coming in contact with the vagina/anus will reduce transmission risk to effectively zero, and condoms don’t break all that often.

Correct condom use will make you as an individual much safer. But as a public health strategy, telling people to use condoms has a significant but limited effect.

More info here: https://www.aidsmap.com/about-hiv/do-condoms-work