Being unfaithful is the end of many relationships.
When things aren't going right, it can be tempting to look elsewhere - especially if you suspect or find out that your partner has been unfaithful to you.
Having an affair might distract you for a while but the guilt could make things even more difficult with your regular partner. If they find out, it could spell the end.
Even if you know they've already been unfaithful, two wrongs will only increase the amount of hurt going around and is unlikely to improve anything.
Meanwhile, a rocky patch in a relationship is probably the worst time to contemplate introducing other sexual partners into your lives - even if you both agree.