Redefining marriage brings about social change. A small part of this change is positive, but it also brings about a lot of negative social outcomes too. In this lecture we look at the core value of fidelity - an essential building block of heterosexual marriage. We find that in a marriage redefined society this value becomes ever-more weakened. Without this core value in place as a social expectation fewer heterosexuals marry and more end up divorcing. More children suffer a childhood without the love of both biological parents - and future society struggles even more with social decay.