No, you don't understand.
The vast majorities of STDs have no symptoms. There is a greater than 50% chance that your partner does not know they have an STD.
By choosing to put the burden of action on the partner, you contribute to a very special kind of fucked up dynamic:
1) 20% of the population gets STDs, go about their lives blissfully oblivious. The mode number of sex partners is actually only one though - among the promiscuous sorts of people you're realistically having sex with, unless they're all rigorously following good testing practices this is really more like 40-80% have some STD of some sort. Statistically speaking, you probably have a few yourself.
2) Why doesn't it seem like everyone has STDs? Because most STDs do not do anything. HPV, herpes, etc are totally asymptomatic in most healthy people - something like 1-5% ever even gets symptoms.
3) Some unlucky person gets symptoms once. Or perhaps they got tested (The CDC doesn't even advise herpes testing except in high-risk cases and symptomatics anymore, in part because herpes isn't harmful enough for them to care about and they've found that knowledge of the stigmatic disease is more harmful than the disease itself). Now, for the rest of their life, they have to go around saying they have an STD and get treated like a leper .... by other people who often have the exact same STD but are unaware of it.
Leading to 4: A fucked up incentive structure such that the less you know about your STDs, the easier your life. Those who are not in the know get all the privileges that come with the confident claim of "I'm clean" with none of the hard work of actually making sure they are disease free.
Meanwhile the hysteria about STDs is ridiculous - people care more about genital vs. oral herpes, even though oral herpes is more dangerous, and no one cares about mononucleosis, even though it's more dangerous than either brand of herpes (all three are permanent). They're only stigmatizing it because it's sexual.
(This is the part where people try to say I'm just rationalization not stigmatizing STDs for selfish reasons. No. I don't have any STDs, except mono and that's technically not an STD. This is what any sane person who is actually educated about this will tell you. Unfortunately most humans are neither sane nor educated.)
Again, most people do not know about their STDs - If your partner count is 5+, and you are using "I trust my partner to disclose", even if everyone discloses everything they know you're still likely to have herpes or HPV, at the least.
"But", you're thinking, "I get tested! The test comes back and I'm certified "clean" and I can go around telling people I'm "clean""!
To which my answer is - did you actually check which diseases they tested for? Chances are they left out HPV. They often leave out HSV as well. This is because they usually they only test for the usually harmful diseases, and not the ones which are generally asymptomatic (because of the price, and the high false-positive rate).
Like you said, it's been "working for you" in the sense that you're happily asymptomatic. That's great - there's nothing actually wrong with what you're doing provided you understand the level of risk you are actually taking. Which you don't seem to. What is wrong is that you claim to have a "zero tolerance policy" and are under the delusion that you're "clean". You don't. You've actually got a super lax policy. If you want to claim "zero tolerance" and "I'm totally clean" to all your partners, you have to specially order the expensive herpes and HPV tests, and ask your new partners to order the same before sex.
Otherwise, you're going to need to accept that what you're doing is not zero tolerance - it's actually "I'm exposing myself to a mild-to-moderate level of risk".
Get what I'm saying? You can keep going the way you are going and admit that you are comfortable with a certain level of risk.
OR, if you'd rather be able to accurately say "oh yeah I'm totally clean and have a zero tolerance policy" to others, at the bare minimum, very least you have to actually ask your partners if they've been tested before sex. (This is the bare minimum and is still not actually zero tolerance, because like I said, the standard STD panel doesn't test a lot of the extremely common and usually-harmless STDs)