Can I get any venereal diseases from having unprotected sex with a guy then with a girl?
Viruses and bacteria don’t care whether you have sex with a guy and/or a girl. If sexually transmitted diseases are present, they are likely to be transmitted by sex. If they aren’t present, they won’t be transmitted; of course the more times you have unprotected sex, the more chance of contracting an STD. At the link are links to information on variouse common STDs.