Just to clarify, what you're saying is that someone who is a lesbian will always be a lesbian, but they might change from liking blonds with glasses to redheads with perfect vision as a preference?
I wish I was better at translating my thoughts into writing. 
Lesbian is a very general description for something that can be quite complex, which is true for every other orientation, too.
It is also not absolute by definition, that depends on the woman in question. Some are firmly in the "women only, forever" camp, others are closer to being sexually attracted to men, too. Clarification: Many people are attracted to persons outside their own orientation but often don't realize it or won't admit to it. It usually isn't important because it has no impact on them.
We change over time. The 18 year old you is probably a different person than the you now. Some things stay the same, some things change completely. If you were already leaning towards another orientation (usually bisexual) then you might find yourself attracted to a person of a different gender as you get older.
I know of one lesbian woman who fell in love with a man and married him. She still identifies as lesbian, with her husband being the sole exception. She's not attracted to any other man but still loves the ladies.
That's just one example of the complexity. I've talked with a couple hundred lesbian and bisexual women... I feel it would be wrong to say "a lesbian will always be a lesbian" but pending psychological trauma orientations usually expand, become more broad... instead of narrowing down. (The "You'll always be a lesbian" was directed at WildOrchid in particular.)
For the most people orientation doesn't change. What changes are the preferences. Preferences can be about certain looks, certain personality traits and sexual, too. That's actually a rather big topic in itself and I'm not qualified to talk about it nor do my personal experiences constitute a large enough sample size to be representative.
Keep in mind that we are attracted to persons, not genders. No one is attracted to every woman or every man.
To get this derailed thread back on topic:
While the vast majority of people worldwide is indeed hetero the boundaries aren't as clear cut as most people believe. There are overlaps, there are fluid areas...
... and in any case, romance options shouldn't be demographic. Otherwise half of them would be for hetero asians. 
To repeat myself: We shouldn't forget that people like to play out different roles. There are many straight guys who play lesbian romances and many straight women who play gay romances. Those are part of the demographic as well and with that it makes more sense to treat every option equally because you're still appealing to the majority of people.