If i had unprotected sex with a woman for argument's sake was hiv positive how likely would i get hiv?

Low risk: The average risk for the male partner from a single episode of unprotected vaginal sex, if the female partner has HIV, is 1 in 2,500. Higher if she is untreated and was recently infected or is untreated with large amounts of virus in her blood; or a lot lower, if low virus in blood, and zero risk if on good treatment. If you think you have been exposed, get tested. And in the future, use condoms!