If both partners don't have stis is it possible to still get an sti because of unprotected sex ?

No: All STDs are transmitted person to person. So if one has an std, they got it from another person, whether it's your partner or not.

No: The problem is that you are seldom certain that your sexual partner does not have a sexually transmitted disease. If they do not, then you cannot get one from another source. When my students ask if you can get these from a toilet seat i tell them only if you are having sex on the toilet.