Ahalya is not tainted as Perumal had relieved her of her sins and this is indicated in Ramayanam. If we even think that they are tainted, the thought itself will beget our sin.
We should take this as an example as to what kind of taints are been mercifully absolved by Perumal, asserts Swami Desikan in many verses of Paduka Sahasram.
They are divine beings; these are instances for us to learn good morals.
Notes
Even today, we see that those who have erred are held in high esteem unlike those who have not erred. Bhagavan grants the relevant rewards or punishment accordingly so we need not worry about it.
Moreover, it is a sin if we think badly of those who have atoned for their sins, so one must be careful in these matters.