Consider the traditional argument against non-marital sex: pregnancy. I submit that while this argument doesn't apply in all cases (e.g., when the woman is already pregnant, or when she's 70 years old), there is a lot to this argument in typical cases:
- It is wrong to take on, without sufficient reason, a risk of being unable to fulfill one's responsibilities. (Premise)
- Non-marital intercourse typically involves the risk of acquiring parental responsibilities that one is unable to fulfill. (Premise)
- Therefore, it is at least typically wrong to engage, without sufficient reason, in non-marital intercourse.
Claim (2) is particularly clear in the case of the man, who in typical cases will be unable to fulfill his day-to-day relational parental responsibilities without being married to the mother of the child (cohabitations tend to break up). But it is also true in the case of the woman, both due to financial and time constraints and because one of one's parental responsibilities is to cooperate in the co-parent's fulfillment of parental responsibilities.
How restrictive the conclusions of the argument are will depend on one's weighing of the reasons. In (1), "sufficient" has to be measured relative to (a) the probability of the risk, and (b) the expected moral weight of the responsibilities one would be unable to fulfill. Now, the moral weight of parental responsibilities is very high. The probability of the risk depends on whether we are evaluating a single act by a person committed to that being the only act of non-marital intercourse (e.g., during a year) or a habit (or policy) of non-marital sexual activity. While the probability of conception from a single sexual act where the woman is using hormonal birth control[note 1] may be rather low, the probability of conception from a habit of non-marital sexual activity is far from negligible. The Alan Guttmacher Institute says typical use effectiveness for oral contraceptives is 92.5% and for the male condom is 86.3%, i.e., 7.5% of female users of oral contraception and 13.7% of women whose partner uses condoms will get pregnant each year. Perfect use effectiveness is higher (99.5-99.9% for oral contraceptives and 97% for the male condom), but it does not seem one can count on one's partner's perfect use. I suppose combining the male condom and oral contraception would result in yet higher effectiveness (and significant protection from disease), but still the effectiveness would fall short of 100%, to a degree such that significant numbers of women would be getting pregnant each year.
However exactly one evaluates which reasons are sufficient, I think it is plausible that when one considers the moral weight of parental responsibilities, pleasure is unlikely to constitute a sufficient reason. Moreover, relational reasons for pre-marital sex are not likely to carry that much weight in light of the fact that if one simply is patient and waits, one is likely (in a monogamous society with a roughly equal sex ratio) to find someone to marry, and then have all the relational goods that one would get from pre-marital sex (if there are any such) to an ampler degree.
This argument is not sufficient to show that all non-marital sex is wrong. But it does apply in many cases. I do actually think all non-marital sex is wrong, but that will have to be established by other arguments.