Is it permissible to revoke the oath I took to refrain from opening anything on the internet related to girls? Would I be considered sinful if I opened websites or pages related to girls that do not contain forbidden things? Can I access religious pages or questions related to girls?

If you take an oath and then realize that something else is better, you are permitted to do what is better and expiate your oath, as stated in the noble Hadith. The expiation for an oath is to feed ten poor persons, or clothe them, or free a slave. If you cannot find any of these, then it is to fast for three days, as mentioned in Surat Al-Ma'idah.