If you lie about who you slept with, and you give your ex an incurable disease, they could file charges against you. It happens more than you think.
Building a relationship on lies... never works. Be honest. If it is something he can't live with, it is better to know right away.
The other thing you should know is that secrets like this don't stay secrets forever. Somehow, the person you are hiding things like this from will always find out.
Attention: NOTHING on this site may be reproduced in any fashion whatsoever without explicit consent (in writing) of the owner of said material, unless otherwise stated on the page where the content originated. Search engines are free to index and cache our content. Users who post their account names or personal information in their questions have no expectation of privacy beyond that point for anything they disclose. Questions are otherwise considered anonymous to the general public.