I'm staying with an SO who cheated
Is this the first time they cheated on you?
Yes, why do you need to know?
Cheating can hurt trust and the ability to trust, and forgiveness is one step you need to rebuild it. If you can forgive your partner, then it helps you as a couple to be able to move forward through an emotional struggle.
My SO don't see it as a big deal.
How do you feel? You are half the relationship, and your feelings should be taken into account. You should be with someone who values you.