I once did a lot of reading on this to try to figure out the same question for my LL in the beginning
What I cam away with is that there are a few different categories of Trannys - the female who wants to be male, the male that wants to be female and the crossdressers that don't want the sex-change, just the "other side of things"
The male that wants to stop repressing the female side and get the change ops, is not necessarily gay, as a mater of fact there are many that are "lesbians" if you use that thought process - they still want to be with women, although many of the sponsors miss this fact when designing sites - they think they all have to be men that want other men - an old "religious upbringing" hold-over maybe?
Females that want to be males are a little rarer, although there is still a market for that type of site - and I have yet to see anyone push that niche - looking at it - it might be one to make some targetted niche money at
crossdressers seem to not have the commitment to the other gender but "get off" more on the eroticism of being in the other sex's clothes - although again some generalizations are made in the sponsor world about them being gay or lesbian - because thats all the sponsors have delved into - again - a niche that could probably be exploited