If I notice my husband's infidelity, my love for him fades away. What should I do about a husband who won't agree to a divorce?

After getting married and being madly in love with each other, once they had a child, the child began requiring more attention than the husband, and the neglected husband ended up having an affair. The husband insists that an affair is merely an affair, while the wife argues that if the husband is going to cheat, then why shouldn't she do the same. The wife may not have had a physical affair, but perhaps her heart had truly strayed.