Clearly, Legend is referring to the lustful aspect of "love/relationships" in this song. The ideas that he talks about are especially common to young people who are newly in love. There are two aspects to what he is saying.
First, it is often important for people in new relationships to show their commitment to the relationship, to "claim" the other person. Usually, this involves holding hands and other sorts of mild public displays of affection. Legend is describing much more extreme ways of doing this.
Second, sex is a major taboo in our society and many people are attracted to the forbidden. People who are newly in love often want to do forbidden things together, partly because they can and partly because the more "wrong" a thing is, the more exciting it seems.
Legend is saying, then, that love and relationships involves lust and that lust can play out both as a way of claiming the other person and as a way of adding excitement to life through the breaking of taboos.