John Q. Christian sees Ted and Alice, the fictitious couple in the OP, living together like man and wife without actually getting married. John Q. Christian has the conviction that couples should get legally married if they are going to live together (he has no idea they aren't having sex, and, frankly, it's none of his business, but nonetheless, it appears they are).
Sometime later John meets Jane Christian and they fall in love. John and Jane, out of expedience, end up living together because that's what Ted and Alice are doing. Even though it's against John's conscience to do so. Ted and Alice have caused John to sin against his own conscience. To him it is a sin to live together like Ted and Alice are doing, so when he does the same thing they're doing he is sinning:
"...if anyone regards something as unclean, then for him it is unclean." Romans 14:14
Ted and Alice, through their freedom, and through their knowledge (they know it's okay because they aren't having sex) have caused John to sin. Paul says it's better to not exercise your freedom than to destroy the work of God. Ted and Alice aren't acting in love toward other Christians by doing what they're doing.
"15If your brother is distressed by what you eat, you are no longer acting in love." Romans 14:15
It's easy to be a Christian...until it's not easy to be a Christian. Sacrificial love is the distinguishing mark between the love the world and how the Christian loves. Ted and Alice need to make the sacrifice of their freedoms in the name of peace and unity and not causing a fellow believer to sin.