Heteropatriarchy means everything that promotes the idea that we should be in monogamous opposite sex relationships. So our culture that promotes this as the ideal way to live, and practical issues such as available housing and legal protection in the form of marriage, that make this the easiest choice.
Valentine's Day celebrates this through commercialisation that urges you to spend lots of money, to show that you really do love that person of the opposite sex you are in a monogamous relationship with.