Are there any advantages to not having sex before marriage? If so, what are those advantages? How important are they when compared to the advantages, if any, of having sex before marriage?
I'll just answer from the mystical Hindu POV, and not get into some of the other arguments (hopefully)
Because marriage is both a mystical union, and a civil union that unites families, in traditional Hinduism it is considered the ideal to be a virgin when one marries.
But you didn't ask whether you meant sex with the same partner, or sex with several partners, so I'll say a bit about both. For the multiple partner part - from the mystical POV, each partner is considered a psychic connection, and each connection has an effect on you, whether you know it or not, so the more connections you have, the more you feel 'side' effects. So when a former partner has an emotional reaction to something, it still affects you, via the mystical connection, like an invisible electric wire. Not all Hindus, let alone all people, would believe this, and the intellect will rationalize it's non-existence. But mystics would believe it. So this is the essential inner reason why promiscuity is frowned upon, as it's not considered healthy for the spirituality of individuals involved.
It is considered also that when virgins marry, the bond is tighter and stronger, so the psychic cord attaching the two people is stronger, not frayed so to speak. An idiom that reflects this is 'tieing the knot'. Of course we're talking ideals here. In arranged marriages, the couple traditionally does not consumate the marriage for 3 or 4 days, depending on regional variances. The first night is spent in the grooms parents house, the second night in her parent's house , and the third night alone, but with a barrier between them. This strongly reinforces the idea that marriage is much more than a sexual union - ie. friends forever, economic union, spiritual brother/sister, parents together, and much more.
The idea on sex before marriage is not considered nearly as much a deterrent to spiritual progress when its just the one couple. Love marriage is getting more common. It would be considered being married on the physical plane, but not on the celestial or astral planes. A marriage informs the inner beings, and that's why Hindu marriages can get complicated, and hard to follow, A lot of it is the priest informing the inner beings, and Gods and God of our religion.