Complex sentence: In rare cases weyrmating is as deep and permanant as a marriage, however usually a weyrmate would be the equivalent to a "boyfriend" or "girlfriend," a form of serial monogamy.
Split into simpler sentences: In rare cases, weyrmating is as deep and permanant as a marriage. However, a weyrmate would usually be the equivalent to a "boyfriend" or "girlfriend:" a form of serial monogamy.