I concur with the majority of the posters here, and it bears repeating that most of us first noticed these urges to put on girls' clothing as young as around ages 5-6. Since that precedes even the earliest onset of puberty amongst humans and the corresponding first awareness of sexual urges, clearly something else is at play here, and it comes down to gender awareness/confusion.
Sure, a temporary sexual component kicks in around puberty, and if I had to guess, much of that has to do with desiring females but not being able to act on those feelings. In this regard, Mother Nature plays a cruel trick on us since teenage girls mature faster than we do and typically go for the older guys at that point in their lives, thereby leaving us hanging (or self-pleasuring, as the case may be
). And so, being the budding crossdressers that we are, we do the next best thing, which is to create an imaginary (surrogate?) girlfriend via our alter egos. But once we start dating, developing relationships with SO's, becoming sexually active as the fundamentally heterosexual males that most of us are, and eventually getting married or entering into long-term relationships, the sexual component starts to wane and we revert back to our crossdressing as an expression of gender identity.
Interestingly enough, these crossdressing urges often start to plateau (or even go dormant) during our early adult years as we start to become more focused on starting families, advancing our careers, and paying mortgages etc., and life suddenly becomes very busy. Then around age 50, these urges come back with a vengeance, and almost to the point of being an obsession in some cases. Maybe that's because most of us have settled down now and our lives are in "cruise" mode at that point, maybe it has something to do with lower testosterone levels that typically occur at that time (andropause - the male equivalent of menopause, but much more subtle in its direct effects), or maybe it is the jarring realization that we have more of our lives behind us than ahead of us, and if we don't explore our feminine sides to the fullest now, time will pass us by. Maybe it is actually a combination of all three factors, but either way, it should come as no surprise that many of us here are also between ages 50 - 65 as survey after survey has demonstrated.
All that said, let me put a different spin on the original question. Many of us get married or enter into long-term, monogamous relationships with the expectation that this step - along with a regular sex life - will cure us of our crossdressing urges - NOT!...which just leads us back to gender expression being at the root of our crossdressing inclinations.