Exclusion for VDs

I am looking over the terms of my company’s medical insurance policy and there is an exclusion for venereal diseases. This seems to be standard for all providers. Any idea why? Of course, this is “self-inflicted” in the sense that you choose to have the risky sex that causes this, but then, there is no lung cancer exclusion for smokers. Is it a moral issue or is there some other reason I am missing?