I kind of get it. But I think your are being breathtakingly naive. Which is understandable given you are so confident about your husband (which is lovely to read).
You say he would be too sensible to catch an std and be careful.
But that’s is the problem. The sex is not usually what devastates. No one find out their partner cheated and is fine with the lies but not the sex part.
the betrayal of cheating isn’t the sex. It’s the lies, it’s the realising you didn’t really know that person at all. It’s realising the person you built a life with and have a future with, has been emotionally and physically (and often financially) investing somewhere else. It’s the thousands of little lies they have told you, so they can have what they want while leaving you in the dark. Affairs often become a form of abuse. Constant lies, manipulation, often gaslighting and cruelty.
People often realise there are huge parts of their partner that they don’t know at all. They realise their partner is a liar and only cares about their own wants. Not what the kids need, not what’s best for the family etc.
So, when you realise there is a part of your partner you didn’t know existed, they have consistently lied to you, damaged your trust and betrayed you, would you be so sure he was careful and used protection. Would you believe he made sure there was no risk of OW getting pregnant.
Because your husband, hasn’t betrayed you (thankfully) you think it’s about the sex. You are also confident that you know him so well you know he wouldn’t put you at risk. But if he was a cheater you would, likely, find you felt like you didn’t know him at all. So all the security you feel now would be gone. Again, that’s parts of the devastation.