There was a time, when this was so simple. You got your ultrasound. You visually saw onscreen, boy or girl. Attaching expectations of their future based on their sex. Only to get it completely wrong- regardless chemical changes within, or environmental/external, you, being the parent have an obligation that you chose to create life, provide for it, nurture it and be responsible for its well being-
We are born to whatever it is we are meant to be, granted you can change your lifestyle but you cant change who you really are. For instance a gay individual will not "choose" to be gay, some of them are just born that way, they feel the same way as a straight person would feel about them, a straight person would not imagine going for a member of the same sex, but a person who is gay or bisexual or transsexual or pansexual or whatever it is you may be, would never imagine a relationship with a straight individual. I mean bisexual means both genders but hopefully people reading this gets my example, especially gay people who are do go for the same sex, they could not imagine going for the opposite sex, they still go through the same feelings, the same drama, the same hardships, the same victories, it just depends but one thing is we cannot help who we love.
I believe that heterosexuals are born that way, the same way that homosexuals are born that way. It all has to do with the genetic make up. People can't choose who they are attracted to. Some choose to hide their preferences, others don't. It is a tricky subject, but my belief is we are how we are born.
Heterosexuality is a very complex subject. Some people have no choice in who they are attracted to. Others choose to be attracted to the opposite sex. Either way, it doesn't matter. There has been quite a bit of research on this subject, and some of the time it is genetically based. Other times it's environmentally based.
Since the days of Adam and Eve, mankind has procreated. Procreation takes the union a man and a woman. Heterosexuality is the natural and typical state of sexuality of the vast majority of people, and the tendency toward heterosexuality begins at birth. Boys tend to be more masculine and girls tend to be more feminine (typically) right from the start. Later in life they are drawn toward each have children together and form a family.
We are all born with our sexual preferences already present. We do not make a decision one day to either to attracted to men or women, we are just attracted. Those who think either preference is a choice need only think make on their own teen years and remember simply finding someone attractive, with no decision make either way.
I would say that it is 50/50, not all identical twin are the same sexuallity, yet the colour of peoples eyes is 100% in a persons DNA/genes. Back in ancient times many more men had sexual relations with other men, not because their genes or DNA was different from todays, but because attitudes were different.
There are a number of genetic factors that can affect the development of a child. Additionally there are a number of external factors that can affect the development of a fetus. Furthermore there are a overwhelmingly massive number of external as well as internal factors that can affect sexuality after the child has been born ranging from culture, religion, environment, and personal preference.