@Fuckthatguy
I 95% disagree with all of that, the shitty emotionally manipulative, irresponsible act is the OPs one night stand
Shitty and irresponsible yes, but it's happened, it's history, it can't be undone, and neither OP nor anyone else has any control over it. What OP is still in control of is what happens from here on in.
and withholding of that information
As I've already mentioned, "fundamental honesty" is noble in principle, but it isn't a basis for workable relationships between human beings. Internet forums, Mumsnet included, tend to be hugely fond of straight forward black or white thinking, with little nuance, and no apparent comprehension of the fact that humans are flawed, they are not black or white, humanity and life exists in shades of grey, and there are times where it serves no worthwhile purpose to be fundamentally, brutally honest.
Try it, but if you adopt a policy of 100% honesty, at least have the decency to operate that policy literally, and then see how far you get. Tell your partner that no, you don't want to watch TV with them because their taste is crap and it bores you to tears. Tell your partner that no, the meal they slaved over for hours wasn't great, in fact, you've had far better on multiple occasions. Try it, see how far it gets you, and how quickly you realise that not only is 100% honesty not a good thing, lies and dishonesty are actually essential for human relationships to persist for any length of time whatsoever.
As a PP has highlighted, this thread is chock full of people wading in as if this is a straight forward black or white situation, a yes/no proposition, when it's an order of magnitude more complex.
but the 5% which agrees is that OP should end the relationship as it’s now tainted and based on a lie
You are astoundingly naive if you don't comprehend that each and every human relationship is based, in part at least, on lies of varying magnitudes. It's part of the human condition. As for being "tainted", it's not your relationship, so that is not for you to judge, only OP can.