I wouldn't stake my life on the Queen never having an affair.
I've been struck recently by the fact that there's an entire PR apparatus that comes both from within and outside the Royal institution that is expressly geared to ensure a favourable narrative is told and a benevolent light shone. What we see is often curated. I saw yesterday, a former newspaper editor admit that they would sometimes not print stories that cast the RF in a disreputable or unfavourable light.
Think about it, the RF has been around for centuries, they've survived that long, even managing to thrive amidst a democracy and their PR game is second to none. It is already going into overdrive right now to recast the RF in a good light so they can continue to survive and it is actually working.
The constant narrative of a great love story between QE2 and PP, that she was besotted with him all her life, and relentlessly and totally bound to duty with an iron clad will with absolutely no wavering is very unlikely to be entirely true or just not true at all. We don't actually know what their marriage was like. We don't. There seems to be rumours of PP having multiple sexual dalliances. If true, (and Lownie implies it is and I doubt he would risk his reputation as a historian) assuming the Queen was also a virgin at her marriage and not sexually pedestrian in her outlook, that has to have hurt at some level, Queen or not, at least initially. Later, a hardening might emerge, but I doubt it at first. She's a human first. Realistically the marriage could not have been happy the entire time. And pragmatically, few people's are, let alone with all the weight of Royalty.
So I wouldn't stake my savings on the Queen never having a 'moment'. And one can also make some off choices and not be any less devoted to ones faith. Redemption is sort of a key point of the whole thing.
Of course there's no evidence of her doing any such thing as having someone else's child. But I wouldn't cast it in stone that she didn't.