Why doesn't anyone say anything like this about infidelity? Most of the marriages I've seen end in divorce have had an affair involved at some point.
You can't point the finger at one thing and say, "THAT is what's causing marriage to go downhill." I don't know if this lady is a Christian, but it seems like whenever I hear about a group opposing same-sex marriage they are affiliated with Christians.
Christians associate themselves with Jesus and this is what Jesus says, "Why do you look at the speck of sawdust in your brother's eye an pay no attention to the plank in your own eye? How can you say to your brother, 'Let me take the speck out of your eye,' when all the time there is a plank in your own eye? You hypocrite, first take the plank out of your own eye, and then you will see clearly to remove the speck from your brother's eye." -Matthew 7:3-5
I've never heard a testimony of someone coming to Jesus because someone said, "You're doing it wrong!" But I've heard many testimonies about people being loved just the way they are and deciding, "I want that too!"
If we want to see change in our culture and strengthen marriages we need to take our cue from Jesus:
Jesus replied: "'Love the Lord your God with all your heart and with all your soul and with all your mind.' This is the first and greatest commandment. And the second is like it: 'Love your neighbor as yourself.' All the law and prophets hang on these two commandments." -Matthew 22:37-40