false logic. You were exposed on a daily basis to females wearing feminine articles of clothing...thus there is your environmental cue. Stevie makes a huge point, that a lot of what TGs do is due to society and artificial standards set upon to divide male/female behaviors. We live in a Fraternal society where the males are "in charge" and do the sweat work. However there are societies in the world which are Maternal (which actually makes more sense since you always know who your mother is, your father not so much). In other societies the wearing of items associated with femininity in our world are often part of the masculine life (skirts, makeup).
So I think it is a lot to do with environment and nurture. Many feelings mix into why people dress here. A want for love and attention and seeing that women get more of that while men have to act without. So there are some here who reflect that in the hopes that they will get the attention from whatever sex they desire. Why do so many fantasize about being with a male while dressed...they want the feeling of being desired that men don't get in this culture. Sexual roles are no longer as rigid and the younger TGs are less likely to have anxiety over what they wear.
If it was genetic, why are there not more stories of men who became women 100 years ago? You know there must have been MtFs out there... and since, as stated over and over here, you can't stop it, how did they manage to not be seen? You may argue that there were less gays 100 years ago but the truth is that there weren't, they just presented differently. Brokeback Mountain wasn't an unusual thing for those cowboys. And those spinster sisters? Often not sisters. Yet (with the exclusion of the Beardache or two-spirit native Americans) one rarely hears of MtFs. So it can be controlled at the very least (shoots the whole "you can't keep it hidden or not dress your whole life" theory now doesn't it?) There are tales of women who lived as men...yet few the other way. If genetics were true then we would be still hearing a greater number of FtMs. Yet, the opposite is true, MtFs are far more common.
There are many things that blend to make the current MtF explosion. Yes there were TSs 150 years ago. Some I am sure became female presenting and never married but lived as women in far away places. Now, with the advent of shopping and the ease of getting clothing and wigs, it is far easier to present. And you don't have to be a spinster.
The perfect answer isn't going to occur. Nature/nurture? Combination along with a few other things like environment and opportunity. You are seeking an answer for which the question really doesn't matter