Well, I started dressing when I was about 3-4 years old, and it certainly wasn't sexual.
It wasn't sexual until I hit puberty and my testosterone levels went crazy. Then it remained sexual until I got old and my testosterone levels waned.
Your brain is hardwired to interpret cross-dressing as actual contact with a female. So when you cross-dress your brain goes into action releasing neurotransmitters: dopamine is what provides the urges to seek gratification, sexual or otherwise (its also associated with compulsive behavior), then there is serotonin which is associated with sensations of well-being, happiness, reduces stress, and improves sleep, and then there is oxytocin which is associated with social empathy, trust, bonding, and love. The testosterone is associated with your sex drive.
When you orgasm your dopamine levels drop and your prolactin levels to up. Prolactin is associated with the pulling away after sex. So immediately after orgasm you may experience a loss of the pleasure with your cross-dressing. Of course if you wait a few minutes your cross-dressing interests will return.
As far as having a willing sexual partner reducing the urge to cross-dress: Well, at one time I thought that when I get married I wouldn't need to cross-dress anymore. It was a misconception. The cross-dressing condition is hard-wired in the brain, and it continues to release neurotransmitters whether or not I have a willing sexual partner.
