Thinking of girls and sex while in 7th standard (approx 12 years old) is natural as you went through biological changes which happens for male between the age of 9 years and 15 years. There is nothing abnormal. Since there are no formal awareness about biological changes and sex, parents and elders thing that it is not good for a child to know about this. Therefore, you would have gone through misconceptions and confusions and learn incorrect information. Just forget about all those apprehensions and lead a good and normal married life (if you are already married) or get married (if not yet). Enjoy good relationship with your spouse both in terms of understanding. Be Blessed.