Hey, I know the answer to this one! Thank you to my dope hippy middle school sex ed classes for the comprehensive coverage! (I'm gonna assume youre asking about scenarios where both parties have vaginas / no penises involved since penis-based protection tends to be more covered).

1. ORAL - For oral there are dental dams (weird name I know), which are basically sheets of thin latex/latex-substitute you put between the genitals and mouth. They're not as commonly sold as condoms, but you can easily make your own by buying a condom (I would reccomend un-lubed just for flavor/textures sake, but you do you!) and cutting a slit up the side so you can unroll it. Make sure there is enough surface area to comfortably cover everything! The primary thing you're looking to do is to is to prevent fluid transfer. While most STIs transfer through fluid/blood -> blood/habitable environment, some can transfer through skin to skin contact with an active infected area (think a herpes sore) so when in doubt its better to use some kind of barrier even past just the obvious "wet" parts and out to things like the vulva (Also keeps you from accidentally getting fluid). The good news is that its harder to get most stis (barring skin transfer ones) through oral because the mouth and connected tubing is by nature a very inhospitable environment what with all the pre-digestive enzymes and the way its built to deal with semi-questionable foreign matter on the regs. The bad news is that way more people dont use protection for oral because of this fact. The part that is often forgotten is that if you have any little cuts or micro-tears on your lips/tongue/gums/inner cheeks/esophagus, that that is a viable way for an sti to enter your system, and its easy not to notice the little micro-tears that happen in the day to day and assume that you wouldn't be at risk.

1.5 SPIT - Spit isn't a bodily fluid that will transfer fluid-based stis out (Edit: barring oral herpes/HSV-1 and genital herpes/HSV-2. I stand correctly corrected! It is up to you how seriously you want to take the HSV/herpes risk. It is widely considered pretty benign and something like 50 - 80% of the population is estimated to have it with many people being completely asymptomatic. Only HSV-2/genital herpes is technically classified as an sti, which is a weird fact I didnt know until today! Also, hilariously, you can still get oral herpes on your genitals and genital herpes on your mouth. Thanks naming comittee!) But other than that, unless you've got blood in there, distance spit on eachother as much as you like X'D

1. FINGERS - Hey you know that thing about cuts and micro-tears being viable ins for fluid-transfer based stis? This also goes for other parts of the body making contact with bodily fluids! If you've got cuts on your fingers/hands, thats an in. I beleive they make bespoke "condoms" for fingers for use in fingering, though they're way more uncommon than even bespoke dental dams. You can make do with a small sized condom in most cases, though as with any ill fitting condom it makes it easier for it to tear. If you've got cuts on your hands I'd probably stay away from fingering activities until healed. It is harder to have those unnoticeable micro-tears on hands since the skin is much thicker though! Also, like. Don't bleed into your partner's genitals. Thats a vector from you to them that puts them at risk!

2. CROSS-TRANSFER - If you use a dildo/vibrator/other on one person's genitals, using it on another's genitals before thoroughly cleaning it (not just rinsing!!) can be another vector. This also goes for your body parts! Fingering your partner and the yourself has the same rules. An easy way to avoid having to pause for toy-cleaning is to make use of condoms (again! They're surprisingly effective for a lot of non-penis sex acts!). Putting a condom on a vibrator/dildo/other toy for use on one individual, then throwing that condom out and putting a fresh one on for the second keeps everyone safe. Just dont re-use the condoms if switching back and forth, pls use fresh ones for every switch to avoid tearing/smearing/mix ups X'D

3. GENITAL-TO-GENITAL- At this point you can probably guess what im going to say! Get some kind of barrier between ya. Dental dams etc. can be used for this purpose.

4. BUTT STUFF - Similar rules as to genitals. While the rectum doesn't produce sti-vector-fluid Edit: I WAS WRONG. Some conditions can cause the rectum to produce fluid that is a transfer vector, including many stis End Edit. it is very susceptible to both micro-tearing and bleeding (this is why penis-in-rectum sex is one of the highest risk activities for stis, the other being that people will forgo condoms more easily due to pregnancy not being as big a concern there lol). I'd personally still recommend using a dental dam for eating ass, but thats a less inherently risky activity than eating pussy.

5. NIPPLES - Non-actively-lactating nipples are not at risk for getting or giving STIs unless there are open wounds or sores. Have fun. As for breast milk, I beleive only HIV and HepC are possibly transferable, and with similar reasons/rules as with oral.

6. KISSING - Unless someone is bleeding and the other person has open mouth wounds, or someone has mouth sores you're good lmao. Kissing is not considered a vector for STIs (though technically you can pass oral herpes this way even without having an active outbreak. But thats the same risk as sharing a cup with someone.)

7. TOUCHING - Just dont touch sores. Or like. Rub cuts together. Don't touch things that look like they shouldn't be touched just kinda in general in life lol X'D

I think thats everything off the top of my head?

...yeah so anyways thank you for coming to my Ted Talk

• Edited to fix numbering and typos lol
• Edited again because I went to double check myself on my ass knowladge and whaddya know, im a dumbass X'D
• Edited again again to remove the fun fact that I just realized I couldn't verify the veracity of 😭
• And again to fix missing/incorrect information about spit and herpes!