Are there any advantages to not having sex before marriage? If so, what are those advantages?
I think an important distinction to make is the time frame being addressed. Today, which is of course what you're addressing, i think:
There can be, if the person is really lucky. For example, if someone thinks there's only one person for them out there, it may be fulfilling to refrain from sex until they meet that person, and it can be helpful in that relationship lasting longer if and when it happens. If the person happens to meet a really good match, with whom they're happy, and it also happened that they met at a not too old age, then that'd probably be pretty advantageous. Of course, one has to hold certain views for this to be fulfilling enough for them, and hold them genuinely, rather than due to cultural pressures. It really depends on the values held and the person's type and amount of ambition.
The above example seems like a pretty huge gamble to me though, and, in my impression, it doesn't seem to be what
would've been the normal hopes and aspirations for most people had it not been for certain culturally and religiously enforced ideas. I'll emphasize more on what i mean in what follows.
How important are they when compared to the advantages, if any, of having sex before marriage?
Not very important, in my view. Whatever measures one has to take in order to overcome their personal circumstances which make them refrain from sex until marriage are most likely no where near as risky or as possibly damaging as the huge gamble being taken above. Which is of course a gamble that does not pan out close to what was originally desired in the majority of the time, and is a straight forward losing bet in a considerable amount of times too. That is, based on my limited personal experience in seeing people's relationships around me and my perspective of things.
I think that if it weren't for certain limitations placed by some cultures and some religions, most people would seek what i think are somehow normal expectations to be had in this regard, much like any other kind of relationship. They'd expect and want to try things out first, have some experiences, grow up with them, have different desires at different stages of their lives etc..
Not necessarily all of them would, like i indicated by using the word 'most'. But i'm talking about generalities. These limitations in my view are always either lacking in reasonable explanations justifying them or outright lacking any explanations at all. If it weren't for social pressure and people being brought up with these ideas, i feel it's reasonable to conclude that most would not ascribe to such incredibly unreasonable and very limiting proposals. Though, these proposals can and do cater for certain feelings, and as such, i think there'd still be an amount of people embracing them.
So, while it can have some advantages, and perhaps in some cases might be even ideal, on the general scale, and most of the time, i view them to be in no position to be compared to having or allowing for sex before marriage.
Now, back to my time frame point, in contrast with the above, in certain past time frames, i think:
These limitations were much more justified and understandable. Due to unavailability of many of the tools that allow us considerable control over the outcomes of the sex we have. Things like birth control methods (aside form pulling out), DNA identification and protective measures from STDs are important tools that obviously were not available in the past, and as such i can so much more easily understand such limitations being placed on one's self at that time.
The only reason i'm wasting your time with this time frame part is in attempt to be fair to the religions that have proposed these limitations, as well as the cultures which continued to enforce them, in the past.