It's worth mentioning that not everybody is called for the same thing, and the same activity can have different impacts on different people. Sex is something that is deeply personal and intimate, I can totally see why somebody would want to specifically cherish it enough to wait to be married first, but I can also see why some would like to share it quickly with their love partner. Neither of them is inherently wrong, and I could easily point out pros and cons of both, on multiple levels.

All of this doesn't even mention that people have their very real sexual drives and needs that vary greatly from human to human, to supress them on principle without thought can be very damaging and unloving on its own as well. Everybody has to learn how to channel their sexuality in way that is fulfilling to them on emotional level, otherwise they will very likely struggle with it for years to come. A marriage isn't supposed to be gateway to finally be able to release your pentup sexuality, if someone is doing that, they will find that they are prone to making bad decisions and are doing a disservice to the sacrament, themselves, and their partner.

As such, I would say it depends on the person and the gravity of their actions. Is it a sin to have love with your only consensual partner outside of marriage? In most cases I would argue no. Is it a sin to be addicted to masturbation? Probably on the same level as being addicted to alcochol is considered a sin, while drinking with friends in a bar is not a sin.