Because sex was intended to be somethine sacred, not something people do when their wasted or so horny they can't even see straight. By waiting until after marriage, you have basically "passed the test" if you will. It just shows that you were able to grow with each other, accept each other's flaws and LEARN to love each other. Ultimately it just shows that your marriage was based on more than sex and lust and actually is something deeper than that: unconditional love.