My answer would vary depending on various factors
The age of my child at the time this was an issue (too young =abstinence)
The maturity of my child and the partner (not mature enough = abstinence)
the length of the relationship (one week? Forget about it)
If I believed my child and their partner were ready to handle everything that comes with and after sex, I would certainly advice using protection rather than abstaining
People are going to have sex. That's a simple fact of humankind, No one can remain abstinent forever, so you might as well prepare them to it in as safe a manner as possible.