I have two points to make. The first is that the sexual act is one of the most amazing gifts God has given us. It brings two people closer together both physically, mentally, emotionally and spiritually. It ties hearts and minds together and helps build unbreakable bonds of trust upon which stable families can be built. It is not the end-all and be-all of a strong marriage, but it is one of the most powerful tools we have been given to use for this purpose. Let me reiterate: sex inside the marriage covenant brings about an intimacy and trust between the two that is unmatched by any other experience.
My second point to make is this. You have to understand what sex is. It is not simply the act of intercourse. Sex begins with flirting which communicates that the two find each other attractive. Sexual tension and desire begin to develop and the two people find ways to be near each other, enjoying the rush of the endorphins and such that flood the brain. When they begin becoming physically intimate, they spend time in foreplay allowing the sexual tension to build. Intercourse is the culmination of this process of bringing two people so close together they are willing to share themselves so completely.
When did the sex begin? Intercourse was at the end, and it was only part of the whole experience.
My concluding thoughts: If you are married, when can you tell your spouse is interested in sex? Often it is when they are playful and flirty with you. Sex starts with the flirt. If you are married, then flirting outside of marriage is off-limits. If you are married, seeking or enjoying sexual stimulus without your spouse is wrong, because it weakens (or destroys) the trust and intimacy shared between only you two. This is why Christ said that even lusting after another is adultery.
But what if you aren't married yet? Well, flirting is ok, because that is how we signal the other person we are interested in moving the relationship in a more serious direction than just friendship.
Now when you do meet and fall in love and marry the true love of your life, do you want to say to them (or do you want to hear them say), "I have already shared this sacred gift of intimacy with others before you. What we have is wonderful, but it won't be unique in my life because of my past decisions."
Sex is ordained of God between those who are married. Flirting is part of courtship. But anything that stimulates you sexually must be avoided unless it is with your spouse. If you don't have a spouse yet, then continue to avoid it.