Sin is a religious concept. If your religion deems it sinful then it's sinful, otherwise no. That is a separate issue from love vs sex.
What is love?
The world has been sold a lie, and that is, there is one person out there for ya, Mr. right or Mrs. right. And is it only they whom you can find true love that works like magic.
That is pure poison leading to disillusionment.
In reality, love is a choice, love is a commitment, love takes work, something most don't want to hear about. No, they just want fireworks going off and great sex and then when it stops, for whatever reason, then they start thinking they have not really found Mr. or Mrs. right and start shopping around again.
Those of faith understand that God is a God of love, and love is the key. The Greeks broke down the word love because it needs broken down. There is love between man and woman, parent and child, peer to peer, and even person to stranger, etc. Of course, they all have various differences, but all have similar themes such as love being a choice. As the Bible states, love is a choice and not just an innate impulse we act upon. However, the world rejects this premise, because they reject God, so what you have is a world that no longer cares much about the concept of love as they have long given up on it, or they forever are searching for it but never seem to find it.
Once man rejects love they then must pursue something. One of those things is pleasure. They go from pleasure to pleasure fulfilling their bodily hungers, but all at the price of forsaking their spirituality and inner need for love. As a result, the world views sex as a mere bodily function, like going to the bathroom, that we all need to respond to in order to fulfill the bodily demands. You do this as you please with no real view that sex can ever be considered wrong or a sin. After all, how can sex be a sin if both get pleasure from it and no one seems to get hurt?
Of all sins, sexual sin seems the most perplexing to people as most get the gist of the 10 commandments, that is, do unto others as you would have them do unto you. In other words, do no harm. But sex, who are you harming if it is consensual?
For one thing, sex is a method to bring life into the world. You are literally participating with God in his plan of creation which is truly astounding and miraculous. This is problematic, however, if the two people are not committed to each other for obvious reasons. But this is just one problem. Another problem are STD's. The more people you sleep with, the more STD"s go around, some of which can kill you. And lastly, if we go around having sex without having love along with it, i.e., sex without commitment, we then lie to ourselves telling ourselves that we love that person on some level, but of course, don't love them enough to commit to them in any way. You then are in a situation that a physical act, which is by design meant to enhance a loving relationship in a committed relationship, is turned into a one way to pleasure yourself with no real concern for the other party. This then gnaws at our inner need for love, which is lacking.
Compare Hugh Heffner to a committed couple who have been married for 50 years. Hugh has had every woman he ever wanted, and the most beautiful in the world. He then ends his life marrying twin girls, every man's fantasy, right? But at the end of the day, there is no real love there. He is just there to get his rocks off as the girls are there because he is their sugar daddy. In reality, they are just hookers. There is no real history to reminisce with the two twins and no committed love. They are essentially strangers who could just as easily empty their bowels in a toilet as they could have sex with each other and then leave them forever without a thought or care about it. On the surface, Hugh has had the ideal sex life, and the overall ideal life of success that all mean crave. But in reality, at the end of his life, he has reduced himself to nothing but an empty shell. His entire life has been dedicated to fulfilling the demands of his body, which eventually fails and dies. Hugh died alone. And once you have sought to please your bodily demands and nothing else, everything you ever did in life dies with it. Conversely, if you pursue loving relationships, your love carries on to the other person you are in love with. And if you have children, your love then extends to them, even when you die. In this way, your pursuit of love is eternal, as where the pursuit of pleasure if finite and fleeing and based in sheer vanity.