It depends on the girl. I think many people assume that only males are sexual beings, but that is not the case at all. During teenage years, girls' hormones are also going through massive changes. Girls also start being interested in the opposite sex. I think boys think about it more often, but girls have similar thoughts less frequently. If a girl is in a long term relationship with a boy and trusts him, it makes it easier for her to be willing to have sex with him. I think girls want to engage in sexual activity, but they first need security and a strong emotional bond.