I remember when I was young hearing a father say about his son: He still thinks girls are just soft boys. He meant that his son was not yet old enough to be interested in girls 'that way', but that statement has always stayed with me. In terms of instant visual attraction, that pretty much describes what 'works' for me...soft boys. Add a little wiggle-room butch/nellie for a more slowly developing attraction. There's already plenty of wiggle room just within the 'soft boys' description.
Dodger has described a continuum related to gender identity and 'manner' all lumped under a few English and Thai terms. Considering the wide range that humans exhibit in combining 'masculine' and 'feminine' traits, that seems more efficient, although still lacking. By defining homosexuality and heterosexuality differently as well (as a physical act rather than anything related to gender identity or mannerisms) that also makes sense.
If there weren't extreme examples of 'masculinity' and 'femininity' (beyond the biological presence or absence of a penis) the concepts either wouldn't exist or would have different meaning. Everybody knows some straight person (male or female) who exhibits at least some personality traits of the gender they are not. I offer 'straight' examples so as not to confuse sexual attraction and sex with gender traits and mannerisms...they are simply not as varied as 'gay' people are.
In seperating gender traits from both physcial gender and physical sexual behavior, the Thai folk-mindset seems a good way to do it.
It is more 'gay' in the western sense of the word for a man to be attracted to lady-boys because of the way we tend to define gender biologically, and include mannerisms as being part of sexual identity...sissy=gay (rightly or wrongly). If Dodger's observations are correct, it isn't as 'gay' for a Thai man to be attracted to Lady-Boys.
None of this takes into account sexual and gender politics which have arisen in the last 40 years to replace common sense and individuality. I think where the Thai have it wrong is that there are more than 3 'sexes' and many more than 3 sexualities. It's the stuff for 1000 PhD dissertations (beyond my ability), but so far this thread has been a good primer for street-level understanding.