Even people who have sex with only one person can get HIV.
Being in love, going steady, or even getting married does not automatically protect you from HIV. You can only get HIV from someone who is infected with HIV, and even then only if you are
involved in risky activities that can spread the virus. But even people who have sex with only one person can get HIV.
There is no risk of transmitting HIV between two people who are both uninfected. The problem is: How do you know? People do not always tell the truth, or do not always know that they are infected. Even if you are in a committed relationship and can trust that your partner is not being exposed to HIV, you should still be tested before having unprotected sex if either of you had any chance of exposure to HIV in the past.
Also, if you were exposed to HIV just a few weeks before getting tested for HIV, the test result might indicate that you are not infected when in fact you are, but a test won't show it until weeks later. So, both you and your partner should be tested once, and after about 3 to 6 months be tested a second time, before you can be sure that you are both HIV negative. This assumes that you both have committed to not having sex with anyone else, and that neither of you has been exposed to HIV since the first test. Remember that if you have a reason to be concerned about a recent HIV infection or have symptoms of early HIV infection, you should let your provider know so that you can be tested with an HIV viral load test that can detect HIV earlier.
Sometimes it is just easier to protect yourself than to keep track of other people. Even people you really like can have things going on that you don't know about.