Is it true that it is less likely to contract hiv through vaginal enter course if you've never had an std or sti?

NO: I am unaware of any literature that suggests hiv is less likely if you have never had an std. People with a history of std are more likely to have hiv. However, for any specific exposure the odds are the same.

Absolutely not true: You can contract HIV without having an std. Having certain stds makes it easier to contract HIV but it is not a necessary condition.