2 things:
1. No, we should not be trying to tell the world how to live. They don't know about righteousness and holiness and "seek ye first the kingdom" and so on, so how are they going to understand it when we tell them that they're not living according to God's rules?
That's Holy Spirit work. Yes, we can plant seeds. Yes, we should witness. Yes, we should evangelize, but that's pretty much our gig.
2. No, the world should not be trying to redefine what it is that God called "holy". Marriage is not supposed to be entered into lightly, but it's supposed to be a covenant between a man and a woman, period. That was God's example and it has not nor should not be changed simply because the unsaved world wants to call the shots and be "equal", but far too long we Christians have done the following: we ourselves have not given proper respect and honor to holy marriage and recently we've begun to cave on the foundations of holy marriage. Christian divorce rate is 50% or more...just like the world...Christian opinion is that we shouldn't have a say in gay marriage...just like the world. On one hand we want to say "God says marriage is to be holy" and yet on the other hand we're getting divorced and having affairs and being abusive and so on...practically handing over our right to have a respectful voice in this debate. The world looks at us, sees the image of themselves instead of the image of Christ, and turns a deaf ear to anything we have to say.
btw: I'm not pointing a finger, but I am raising a hand as "guilty".
So, what do we do about it?