2. Doing it unprotected. Same idea as the mistake #1: The first time you sleep with a guy you probably know very little about his sex habits. Depending on your relationship, you may not even know if you’re exclusive yet, if he’s ever had an STD or if he has reason to suspect he might. Having unprotected sex the first time you sleep with him is always a mistake. If the relationship progresses, you really trust him, and you think it’s really worth it, then you can decide to use other methods of protection (e.g., just birth control pills). But the first time you have sex, even if you’re using an IUD or hormonal contraception, always, always, always use a condom.