Freedom of religion is the freedom to believe what you want and to not have the State impose religious beliefs on its citizens. Even if a majority of the citizens have a particular religious commonality, such as believing in Christ, they don't have the right to make a Buddhist share that belief and more that a Buddhist has the right to force Christians to believe in reincarnation. We have the right to liberty and to privacy and to be human beings and to express our human nature. We have the right to get married, have sex, and to reproduce. Generally, any human activity that is our human nature is, or at least should be recognized by the State as a fundamental human right.
Sexual behavior is in our nature and is an integral part of being human beings.
Thus any laws that restrict sexual behavior must be construed narrowly so as not to violate our fundamental rights to be human beings. There are legitimate reasons, for example, for the State to prohibit sexual activity with children. This is because the sexual activity of one person acts to the detriment of another person, a child, who is not capable of understanding for the purpose of consenting to sexual activity. The minor child is also not capable in many cases of refusing the sexual advances of the adult and it is necessary for the State to step in and say No in behalf of the child. But beyond protecting children from sexual activity and protecting people against non-consensual sex, and public health issues, the State is required by the Constitution to not interfere with the individuals' personal sexual behavior.