As far as unprotected sex is concerned, sucking an apparently clean and healthy dick, meaning no noticeable sores, lesions, warts, discharges, etc. is the lowest risk of any insertion form of sexual intercourse. This is followed by having your dick sucked, having your pussy eaten, eating pussy, vaginal intercourse and lastly, anal.
The reason sucking cock is lowest is because, other than a noticeable issue such as genital warts, herpes or a discharge which should keep you from sucking that dick, saliva and stomach acids will most likely destroy any nasties before they have a chance to infect you. Notice I said "most likely". Nothing is 100%, but it has been shown to be the safest form of unprotected insertion sex so far. Keep in mind that any cuts or abrasions to the gums and inner mouth from something as simple as brushing your teeth too hard increases the risk of fluid-fluid contamination.
Eating pussy is more risky because of the possibility of unseen nasties like vaginal warts or herpes sores that are difficult, if not impossible, to detect as they may be up inside the vaginal canal.
Using condoms is a good idea whenever having sex of any type with someone you are not fluid bonded with, but they are not 100% safe either as they may rupture, but even if they stay intact, they don't cover all of the area of your body that may come in contact with the other person. So even though they may prevent the usual fluid transfer between partners, it is possible for something like herpes to be transferred via skin contact in the areas that surround the genitals.
We still need more info on this so we have an idea how to avoid STDs. I knew a lady that was a call girl and she often would suck on men with a condom on. she said it worked fine. there are some condoms better than others. She said she would not go bareback with anyone unless she knew their history or spent more time with them. Hookers in Vegas , and I am sure in other places have told friends of mine that they dreaded the monthly check at a clinic, worried if they came up with an STDs, like HIV ,etc. As careful as a hooker is, they may throw caution to the wind with a client and go bareback, and make herself vulnerable to a disease. It is risky behavior, and no fantasy is worth dying over or getting a terrible STD. Not to mention the fact they can pass it on to their husband or boyfriends. Be safe in real life but enjoy the fantasies as much as you like.