IIN to be disgusted by the idea of head? (NSFW)

This reminds me of a story I once heard - maybe an urban myth, but very possibly true - about a guy realising that the woman who was going down on him was actually blowing into his dick like she was trying to inflate a balloon. There's also a story on Reddit where a guy confesses that when he was twelve, he used a straw to blow into his urethra, inflated his bladder and very quickly desperately needed to piss. When he got to the toilet, he experienced a dick-fart.

In any case, as long as the person giving the (obviously, for some, confusingly named) blowjob knows what they're doing and isn't trying to inflate your bladder, there's zero chance that any significant amount of saliva will get into your urethra. Even if some does, it won't get any further than the very end. When the penis is erect, the tubes that fill with blood on either side compress the urethra that runs between them. This is why many - maybe most - guys find it very difficult to piss when they have a solid erection.

Your concern isn't entirely irrational, though. It isn't a great idea to introduce the random bacteria that will be in another person's mouth and those that will be on the exterior of your penis into your bladder. Men tend not to get bladder infections because our urethra is long and any bacteria that get into the external end tend to be regularly washed away when we urinate. Women need to be more careful about their genital hygiene because their urethra is relatively short, and it's not too difficult for bacteria to make their way up it to the bladder.