Seat belt is a good thing and effective in reducing
death and injuries in car accidents. But if you
drive less carefully because you think you are wearing
the seat belts, then effect may become negative.

You can have a math model.

Say probility of being infected with HIV is p1
for an intercourse with condom and p2 for an intercourse
without condom, and p1<p2

A person who does not use condom have n times of intercourse will have the probility of not being
infected is (1-p2)^n

A person who does not use condom have n times of intercourse will have the probility of not being
infected is (1-p1)^m

n amd m may offset p1 and o2