Anything is forgivable. It's a choice we each have to make.
However, that does not automatically assume you restore the relationship to what it was, before the offence. That part is up to each individual to decide.
Some have been cheated on, forgiven and reconciled.
Some have not been able to reconciled, even though they have forgiven.
Personally, I haven't gone through an unfaithful partner dilema, so I'm not sure which way I would go, but I would definitely forgive!