The more times you have sex with people who have AIDS the more chances you have to catch it. Yes, if one would have unprotected sex everyday with the same partner who have AIDS, they have better chance to catch it then someone who have sex with a bunch of people who might or might not have it. However, who would be as stupid to have unprotected sex everyday with the same partner knowing they have AIDS?!
In a responsible relationship, couple would start by having protected sex and then, when they make a serious commitment to one another, they should both get checked for STD before giving non protected sex a shot. Some couple I know
never make that step, cause they don't trust one another completely or they don't mind condoms or they want to have the chance to have sex with other people or they use it to not get pregnant.
Every time you have unprotected sex with a stranger you take a chance, HIV is one of the risks, but there are a lot of other STDs out there, do protect yourself. Yes, it's less fun but you know what?! It's still fun and you have the peace of mind of not dying slowly from a horrible disease.