I share some of your fears and can understand where you are coming from. Whilst it is crucial that safer sex is promoted, I think a culture of fear is being created meaning some people feel guilty/scared about every encounter. So here are my thoughts...
Everything in life carries a risk. I mean, every time you cross the street, get in a car or get a flight there is chance something might happen. But you cannot hide indoors.
Sex can be amazing and, yes, every time you have sex with someone you are taking a risk. Safe sex does not exist. Safer sex does. There are ways though to be as safe as possible.
Unprotected anal sex with multiple partners for example is dangerous. Very dangerous! It is also the most common method of transferring STDs including of course HIV. A lot of these diseases are hard to catch - don't make it easy, so always use a condom.
A lot of men make the statement "I always practice safe sex unless I'm in a trusting monogamous relationship." This is not a rule but a choice. I have never had unprotected anal sex even in a relationship. It's not that I haven't trusted my partner but I am entitled to my own boundaries as to what I will and will not do sexually. So if you're in a relationship with a guy and he wants to bareback and you don't, you still say no. You must never be forced to do anything you don't want to. If your partner doesn't like it, that's their problem
What you do with oral is also a personal choice, it is far less risky than anal but it's your decision if you let a guy come in your mouth. Again, don't be forced!! My belief (I'm not a doctor!) is that it makes no difference whether you spit or swallow. The transfer of a disease will be through a cut/sore in the mouth. The choice of what you do will be yours.
Please remember most people are sexually active and go through life without catching anything. If you are safe as you can be you will reduce any chance of infection. Some people are just incredibly unlucky. That being said, the people I know (male, female, gay, straight) who have had an STD have always caught it from unprotected sex. Thankfully it was never anything too serious.
It's good you are aware of the risks and not taking the "it'll never happen to me" approach. Be as safe as you can and you will dramatically reduce the chances of catching anything.