Why do you keep bringing that strawman to which I replied already? No one is talking about feeling identical.
They were not -precisely, they felt that it would have been them, but different (as in disease-free). Not identical at all.

"They would never forgive you, if you lied to them about that. I am a foster mother of two, btw."

Who's talking about lying, something I never do, much less to my sons?
As far as I know, I am the biological father. It's also not something I'd ever go and test as I find it completely irrelevant.
Hell, in our case it's even possible that the elder one has a different biological mother than we think -he is an IVF child and there is a possibility of mixing-up the embryos. If we learnt that it was the case, we wouldn't hide it. But it certainly would make no difference to me, as it would make absolutely no difference to me to learn that my father was not biological.

There is a chasm between not mentioning and adoption (in which case you are certain, by the way) and not mentioning an affair. If the fertility problem was solved because my wife had fun while I was away on business, well all I can say is I hope that she made sure he was HIV negative and that the earth moved for her, but I don't see that it would be any of my business. He's my son, period.

As to never forgiving, well, I happen to know someone who learnt at 30 that she had been adopted. Now, she certainly did not like not having been told earlier, but I saw her with her mother (her father is now dead, alas) and it's clear that forgiving did not take long at all.

Earth provides enough to satisfy every man's need, but not every man's greed. Gandhi