"how can you fully, properly, feel you can "100% trust" someone who is wilfully withholding something from you"
Do you tell your partner absolutely everything - the colour of your snot each time you blow your nose, how many times you wet yourself when you were five, the number of postboxes you walked past in 1996? There are plenty of things that people 'withhold' from each other - not because of shame, not because it's wrong, simply because it's not relevant.
"take the countless threads of late about wives being suspicious of a DH/DPs texting or emailing"
Doing something in a relationship is not the same as doing something before a relationship. This should not be a difficult concept to grasp.
As for the health argument, 'how many?' is a useless question for that. You could have slept with 500 perfectly healthy people, or with only 1 who had every disease known to modern medicine and a few they've not discovered yet. If you want to know for health reasons then the only relevant numbers are 0 and not0 - and even then quite a few 'sexually' transmitted diseases (HPV, HIV, hepatitis etc) can also be caught in other ways.
And finally the 'it makes me feel ill therefore it's wrong' idea - plenty of things make me feel ill: the squeak of polystyrene and the idea of sticking my hand in other people's mouths for a living, for example. It doesn't mean that dentists and people who design, manufacture and assemble the packaging for televisions are all morally bankrupt.