As you are young and therefore probably remember high school, i hope you can better visiualize what i describe now and therefore better understand, why two people consenting is not enough to avoid problems. And this example should just demonstrate, how problems with two consenting people having sex can arise, it is by now means exhaustive and therefore sufficient to develop complete moral norms.
Starting around age of 15 interest in sexual activity arises, high school finishes around 18. Average school size might be around 100 pupils per year.
If it would be no problem for two consenting people to have sex, it should be no problem for all 100 to engage in sexual activity regularly, as there is supposedly nothing wrong about it and it can be fun.
So the 50 females of a hypothetical standard school have until age 17 more or less contionusly sex for 2 years.
Looking at pearl index,
en.wikipedia.org/wiki/Comparison_of_birth_control_methods#Comparison_table
,that means the following:
If they all use condoms for contraception and have typical failure rates, 50*(1-0.85^2) = 14 will give birth to a child before finishing high school. If they received excellent training in contraceptive application (which i think is what majority thinks would solve problem of teenage pregnancies), 50*(1-0.98^2)= 2 will give birth to a child before finishing high schools.
If they all use the pill for contraception and have typical failure rates, 50*(1-0.92^2) = 8 will give birth to a child before finishing high school. If they receive excellent training in contraceptive application (which would have to include excellent training regarding abuse of drug and alcohol, which society fails to achieve since dawn of time), the probability for 1 giving birth to a child before finishing high school is about 30%, so for every fourth school, there would be one pregnancy.
So assuming abortion is not a possibility and assuming the miracle of succesfully teaching teens to correctly use the pill and to avoid alcohol abuse does not happen and assuming giving birth to a child before finishing high school is a bad idea, the advice for teenagers must be:
Do seldom or preferably never have sex.
(And looking into the high school pregancy rates, which are about 13 per 1000 or in our numbers 0.3 per 50 females, it seems teenagers for whatever reason already live mostly along these lines (Or all take the pill with perfect use or a combination thereof). Otherwise the pregnancy rates would be about 10 times as high.)
And thats what we can conclude without ever looking into bible or church teaching, just by assuming that abortion is not preferable and becoming mother before finishing high school is not preferable either.
That the church teaches limitations beyond that is simply due to the bible having limitations beyond that. But we can already conclude that even with modern contraceptions, there would be ample reasons to limit sexual activity in respect to age, stability of partnership, family situation and so on. So that the church teaches limitations on sex is perfectly sensible.
If we now assume that contraception is also wrong, churchs teaching is nearly an unavoidable conclusion(*). Because with regular sex without contraception the probability of pregnancy is 85% per year and children are far better of with married parents, so one should marry if one wants to have sex, otherwise a third party - the child - will be hurt.
So either marry or abstain.
(*Ok, the loop holes of postponing marriage until pregancy happens or using NFP would still exist by looking in this narrow way at church teaching, but both options do not look that sensible and are certainly no sensible option for one night encounters.)