alt Hacker NewsNot really, in most cases! I read up on the per-exposure chances of actually picking up HIV a while back[1] and was shocked at how low the risk is, especially for heterosexual men (4 in 10,000 per-exposure; insertive vaginal sex with an infected partner).
Even in the highest-risk scenario (receptive anal sex) the risk is ~138/10,000 per exposure.
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Most people have sex more than once.
If your risk is 138/10000 per exposure and you have sex with your partner once a week for a year, that's about a 50% chance of infection overall. (Assuming the infection risks are independent, which they might not be.)
If it's 4/10000 per exposure then that's a 2% chance of infection after a year, which for sure isn't huge but I personally would take an annual 2% risk of getting HIV quite seriously.
(Obviously the "once a week" figure is an arbitrary choice for the sake of simplicity, and I am well aware that some people have a lot more sex than that and some have a lot less.)
pregnancy would seem rare too if you only looked at the averaged per-incident chances (swings from 0% to 33%ish for about 2 days for a perfectly healthy breeding-aged couple); or if you were to compare the amount of heterosexual sexual encounters overall to the amount of pregnancies.
but that's not really how the risk should be considered.