The sore in the mouth could be a great many things. People with AIDS who have had multiple sexual partners are at increased risk for syphilis (yes, you can get it in the mouth, which is why unprotected oral sex is risky), herpes, hepatitis, and a slew of others. If I haven't upset you enough, here's a link to other opportunistic infections.
http://www.aegis.com/topics/oi/
That said, I don't know of a polite way for you to say, "I don't want you to infect my kids with anything."
Oh, and Shani, I understand your point, but IMO some of the more outspoken gay activists, by making it a political disease, have succeeded in casting suspicion upon themselves. When it's politically incorrect to say ANYTHING bad about a gay person, it's not a huge mental leap for the average person to wonder if he's hearing the whole story about the likelihood of AIDS transmission.
Of course, if you believe everything the government tells you ...