That's what a nurse at the sexual health clinic told me. Happy to see any stats on this though.
I think it's because of the length of time it stays in your mouth - it's longer if you spit, which means if there is an infection present and you have any weak points in your mouth you're more likely to catch something. I don't imagine it makes much difference to gonorrhoea in your throat, but I really don't know. I don't think I ever had the conversation at the GUM, but that's probably because I could no more swallow than I could fly around the room

That said, when dinosaurs roamed the earth and I did offer OWO I also did CIM despite (and partly because of) it making me heave - the alternative was having it spraying around everywhere and I couldn't stand the thought of the mess