The answer to this is yes this do happen but sex is only one way cats can get infected other types of close contact can spread diseases too.
- Feline herpes can be transmitted by body fluids.
- FIV cat aids can be transmitted by body fluids.
- Feline chlamydia can be transmitted by body fluids,
- Panelukopenia virus can be transmitted by body fluids.
- Calcivirus can be transmitted by body fluids.
- Catflu can be transmitted by body fluids
- Rabies can definitely be transmitted by body fluids and not only to other cats but to all types of animals including humans.
I assume there is several other types of illness that can be transmitted by body fluids not only from intercourse but from bites too.
Source for some of it http://wtvr.com/2013/04/30/cat-stds-on-the-rise/
Source two my cats vaccination card the standard vaccines given to cats here in Norway.