Cheating

I am so against this topic but i want to know other people's opinions. If you are in a long term relationship with your partner and they go to a party and end up a lot more drunk than they thought they would be and end up being took advantage of by someone who was sober but only just kissing them is that their fault? Would you be able to understand and forgive them and not throw away years of your relationship down the drain over one drunken mistake that wasn't in your hands??