With the institutionalization of Buddhism you can probably find some sects
that will approve and some that won't approve. The heart of the Buddha's
Teaching centers on knowing the truth of dukkha, knowing the origin of dukkha,
and knowing the cessation of dukkha. He offered the 8 Fold Path as a guide to
the cessation of dukkha. www.accesstoinsight.org/ptf/dhamma/sacca...

Here is the guide I will offer you. Does premarital sex cause you to
suffer? If so, then it should be abandoned. If not, then it is time to go on to 
life's next question. This is a most personal realization. Your partner should ask
the same question. Of course problems arise when one partner answers yes and one
answers no.

Wishing you a small tranquil day,

Bob

The lay Buddhist generally adopts the five precepts which includes avoiding "sexual misconduct" which for the lay person is defined as (allowing the equivalent for females):

Assuming that by "premarital sex" you obviously also mean consentual sex, the rule is explained as follows: "He avoids sexual misconduct and abstains from it. He has no intercourse  with such persons as are still under the protection of father, mother,  brother, sister or relatives, nor with married women, nor with female  convicts, nor lastly, with betrothed girls." --and it should be noted also includes cases of coercive sex but the blame in that case lays with the rapist not with the victim whereas any consentual case of the above "it takes two to tango" and blame lays with both.

Bob has pointed out the basic principle, that if it causes (undue) suffering, better to be avoided.  I'd like to add that I think the rule embodies the principle addressing mainly the problems of what happens with regard to any child that results, and also possibly with regard to jealousy.  Buddhism doesn't seem to be too concerned with marriage as a sacrament germaine to its religious goals, though it seems to realize that marriage involves vows and to break vows taken is immoral, so marriage is otherwise respected.  Buddhism also encourages couples to live harmoniously with one another in Dhamma.  Obviously if you or your partner begin to feel that your sexual relationship is estranging either of you from Dhamma, you might want to reconsider your mutual priorities.

So because for Buddhism marriage is not really a holy sacrament in itself, consentual premarital sex, strictly speaking, is not forbidden in itself for laity (it is for monks), but even beyond your sense of Dhamma, there are some practical things to carefully consider.  The following is just my own take on it, but it may give you food for thought. 

Even if you use protection, it is often still rarely possible that a child may be conceived (condoms can fail, rhythm method can have mistakes, etc). So if after serious consideration of even the remotest possibility of this, you are not prepared for it, it is really best to abstain, because the one in a thousand chance is going to happen to someone, why not you?  I know from experience that in many relationships this is not easy.  I've been in situations where I knew neither I nor my partner were seriously ready to have a child but for each other's affectionate pleasure we still indulged.  Even though we took care to prevent pregnancy for that reason, it was still marginally possible the measures would fail to prevent it.  So I confess there that that was probably "sexual misconduct" in my case. Nobody blamed us because our society is more permissive now, but I still knew there was an element of wrongdoing in it.

Further, the explanation of "sexual misconduct" above offers many cases that someone else is still responsible for you or your partner, such as parents or relatives, the state (in the case of convicts), or a husband/wife or a fiance (not to mention the jealousy).  In those cases any child that comes of your union is a responsibility not just to you to but also those third parties, whether they like it or not.  So in a way it is like taking a loan you know you can't repay; a burden shifting which is I have to say immoral.  I realize this is a grim view of parenting, but it's a realistic one.  The real joys of parenting also bring real sorrows and pains we feel guilty about recognizing, especially when burdens fall on people not responsible for the creation of the child.  

So, to sound like a stuffy old high school teacher, make sure you are ready for the responsibilities and that you aren't potentially burdening anyone else with your mutual choice, or make sure there is no chance of a child (we all know many ways to pleasure each other that can't produce a child).

in friendliness,

V.