I'm with you, all my friends say she cheated on Butcher but the show leaves it up to interpretation:
After that, all we have is Homelander's speech about how she did it willingly and came 3 times or something, and we all know Homelander isn't the most trustful character. Her reaction at the end is one of someone who's scared/confused. Not only is she seeing Butcher for the first time in years, she's also seeing her potential rapist coming back into her life. If she did cheat then she's seeing both her husband and her lover in one place, which doesn't bode well for her either.
Either way, we won't know what really happened until next Season.