Ok so firstly we’re accepting that the Christian God exists. The next conclusion in regards to your question must be that the age in which people live does not matter. God makes rules that do not change with the times, there is just wrong and right. Murder will never be acceptable in a society, and if it is, that society is going against the will of God.
The tougher, and more interesting, question you asked is why is having sex before marriage wrong? It is not to punish us or anything like that, it is a sin to have sex before marriage because sex is something that should be done to create life, and it is something to create a strong union between two people. Sex is a good way of strengthening the bond between two people, which is ideal for family life, to be able to raise kids in a stable and loving environment. Premarital sex can lead to hard feelings, children that have an unstable family life, these things are the opposite of what sex is supposed to be; therefore, it is a sin to have premarital sex.
Why birth control is a sin is an easier one to answer, it’s the prevention of life, that’s why it’s wrong according to God.
