Because it was expected of me.
I was raised on the Swarthmore College Campus - very Quaker and outwardly very liberal but inwardly very critical and full of heavy handed expectations of what a proper, well mannered and well educated person should do and not do. The societies method was to profess consent, tolerance, understanding and support but then privately underline it all with the "but realize" the difficulties, the prejudices and near impossibility of being successful (ie: financially secure). I understood this all to well by pointing out those who were gay, bisexual and trans which was rather difficult at first - being the early 70's, hence no computers, internet, etc.
I was already different having started college courses in 6th grade and attending Swartmore College for 10th through 12th grades. In some ways it was good because I could be different and be accepted because of my differences. But not that different. So I dated girls, lots of girls, mostly college students starting in 8th grade. In those times too it wasn't so much dating as it was just fun and very open. So for about 12 years I was dating, hanging out with mostly girls and all the while pretty much tripping 24/7. These were the days of acid and mescoline and I was into it.
After that I pretty much mellowed out, started working and was kind of a loner, taking months off at a time to travel. When I decided to go back to college - to actually get a degree instead of taking what the pupeteers wanted me to take - I stopped virtually all partying and got serious. Shunning the control of Swarthmore I switched to Temple and without even thinking about it went right back to the gender conformity instead of embracing the times and independence and transitioning then.
I met my wife in my senior year and three years later we were married. In a way it was the biggest mistake I ever made but it was also the best thing I've ever done - because of my kids. I have three - one is 20 and in his second year of college, one is 17 and graduating high school this year and one is 10 in 5th grade.
So, with the advantage of hindsight, Why did I get married? Expectations, heavily inbred conformity, fear of not being accepted, fear of not succeeding in life. It was easier to do what was the norm than to fight. Being raised Quaker and being taught almost from birth on how to an effective activist and how to fight the long battle for what is right should have prepared me well to make the decision to be true to myself and transition at Temple, but it didn't.
PS
Off the point a bit, but important. Fast forward to today.
In the slow and methodical process of seperating, eventually divorcing but only after making sure that the family is secure, college bills can be paid now and in the future, houses paid for, etc. etc. etc. My wife thinks it very strange and is less than accepting and will be even less agreeable as I start to present as female.
My son, the oldest, highly intelligent having skipped three years of college and got early acceptance to Medical School is surprisingly the least accepting. He doesn't hate me, but hates what I've done and calls me a 22 year liar. I have to concede that point to him. My daughter - the senior in high school grapples with it sometimes but is far more accepting of the situation. My youngest, well she is still confused and has mixed feelings.
As much as they think they are prepared for when I start to present - I know it will be a tidal shock-wave sweeping through and won't subside for sometime. I take comfort in that being another year off though.