Insight: This is quite a question. The answer you would find our culture is promoting is that sex is a normal facet of any romantic relationship, I disagree. Our culture also promotes the belief that marriage is just a piece of paper and doesn't really mean anything, once again I disagree. Our culture promotes the belief that love is a fleeting emotion and people fall in and out of love all the time, again I disagree.
The Bible is quite clear that sex outside of the covenant of marriage is a sin. The covenant of marriage more than a commitment. A covenant is a sacred agreement between God and man. A marriage covenant is the agreement to be united as one flesh with one another until death, this is an agreement between a woman, a man, and God. Therefore, sex outside of this covenant is sin. It is that simple. The Bible is also clear that love is a choice, not just an emotion. When two people get married they are making the decision to love one another and only one another for the rest of their lives. Jesus commands us to love one another multiple times in his teachings, if love was only an emotion could it be commanded? No.
Now you asked me what my thoughts are, and this paragraph is my answer to that question. From my observations and my own experiences, I can tell you that sex creates a bond between two people and this bond is not easily broken. Sex is meant to join two souls together and when people have sex outside of marriage, this bond tears at the heart and leaves parts of your heart behind and takes parts of the other person's. It is messy and painful. I do not believe that sex can ever be considered casual, I believe that at some point some people's hearts are so tattered, they think to themselves 'What is one more rip?' This is how it gets devalued. In my observations I have seen wonderful people feel like they were worth nothing because they had sex with someone and then that someone walked away. Wonder why depression is rampant in our world, look at what we value. We put more stock in relationships with each other than our relationship with God. Try the reverse and see what happens, you might just be pleasantly surprised.
I strive to help others by being a safe place for others to ask questions and pose issues they face.