my husband is a sex addict who thinks it's "no big deal" (yes I know, the same response you get from any type of addict)
how can I get him to take this issue seriously? It is destroying our marriage.
You can leave him for a while to make him realize what will happen if he goes on with his sex addiction.
Getting people to change, when they do not themselves want to change, is difficult. Many women set out to change their husbands, few succeed. You may succeed in getting some small stepwise changes by praising your husband and giving him appreciation when he does what you want.
Possibly you might reach a negotiated agreement, where you both give and take.
Read more about negotiations.