Well actually, they could both have behaved very differently, given the fact that they were committing adultery and Alexander seduction, which went counter to everything which both must have been brought up to believe was the moral way to behave. Dragging God into it was simply attempting to palliate the offense and make it all look much nicer by claiming that such an overwhelming passion had some sort of moral basis. However, I agree the pressures on a young girl must have been immense and at the end of the day, Alexander was simply too powerful and at least in his office as Tsar, charismatic, for someone without a lot more character to resist. It doesn't make either of them very likeable, but there's no evidence that Catherine was insincere in her love for Alexander.