We have enjoyed playing with a couple for many years. He recently had prostrate surgery. Besides not producing any fluid he no longer can get an erection that would enable him to keep a condom on. We have always used protection when playing The last time we played he requested going bareback. I said no and he excepted my decision with no futher discussion. We talked about it on the way home. Obviously pregnancy is not an issue. I'm too old for that anyway. But not too sure about STD's. My husband doesn't think HIV would be possible but I'm not so sure. He said if would be less likely than oral sex withother men. Other std's,while no fun, are not life threatening.
Transmission of hiv during oral sex with anyone is extremely rare unless the receiver has serious gum disease or sores in the mouth.
Hepatitis B and C are other life threatening diseases that can be passed sexually.
Since this is a couple you've played with for a long time I'd ask them to check with his Dr about the risk since he produces much less ejaculate.
Personally if this were my situation I'd probably agree to go bareback if you and he had tested negative, he doesn't have sex with men, and they and you use condoms with their other partners. It sounds like the risk is very low in this situation.