|ginnerobot / Foter / CC BY-SA|
Firstly, a good many werewolves are described as acting and reacting in a fully human way when in human form, and even having children with human partners. I mean, if you want to make your shifters super immune to all disease, that is fine--but that has nothing to do with the species barrier. If human x shifter matings are barren this makes a stronger argument for a species barrier, but that has very little to do with disease transmission.
You see, a majority (60%) of pathogens effecting humans cross species boundaries. Rabies, plague and the common cold come to mind. In fact, it is estimated that a full three-quarters of the most recently emerged and potentially emerging diseases are zoonotic, consider: HIV/AIDS, Arbovirus, West Nile virus, Mad Cow disease and H1N1. And yes, there are sexually transmitted zoonoses (such as the bacteria Q fever and Lepto) although data on this is difficult to collect and it is hard to separate the respective roles of sexual versus non-sexual contact.
Long story short, if you want your werewolf nookie without safer sex precautions, you need a better reason than the idea that diseases refuse to jump the species fence, because they actually do it all the time and most readers probably know that.