A short message suitable for a wedding card is "Thank you for inviting us to share this joyful day! We wish you a lifetime of happiness." If your intent is to add humor, then write "Our marriage advice: Always kiss the cook, whichever cooks DOESN'T clean!"
Heartfelt yet simple messages include "Wishing you a lifetime of love and happiness" and "Best wishes to you on this wonderful journey." Writing "May the joy you feel today last a lifetime" is also a good way to express your best wishes for the couple. A thoughtful and powerful message inspired by the Bible could be appropriate for some couples, such as "Love is patient. Love is kind. Love never fails."