TheGreatGodPan said:
...except for the part about current statistics, because that's basically all we have...
The entire argument does hang on that though.
There's nothing "obvious" about lack of attraction to the opposite sex reducing reproduction.
It's quite possible to reproduce without enjoying it (just as someone can eat lettuce without necessarily liking it). It's also quite possible for societies to put pressure on people to reproduce, or to hide the fact that they aren't interested in the opposite sex.
Since the historical attitude (AFAIK) of society to homosexuals has been completely different to the current attitude in the states, it's just ludicrous to use those statistics. Even where it was accepted (e.g. ancient Greece), it was expected that people would also have a family and reproduce.
A lack of attraction to the opposite sex clearly decreases one's own reproductive fitness
In isolation from society, and assuming that people had no innate desire to reproduce (rather than have sex), that would be true. These are not valid assumptions.
that's why we find the opposite sex attractive
Initially, yes. That doesn't mean that removing the initial reason will remove all reasons which have since come to exist - e.g. desire to reproduce, desire to bring up children, tendency to fit in with society etc. etc.
and an attraction to the same sex does nothing to increase it
Baseless - chicks dig gay dudes.
Nobody has come up with any mechanism by which gays increase the fitness of their kin (they would have to increase it a lot to make up for their own lack of children, basically spending all their time in an attempt to make their kin more fit).
Not true, since:
(1) They probably did have children in most societies until recently.
(2) The total amount of new children a tribe/society can support is usually lower than the total amount the women in that tribe/society can produce: if gay men don't have children, there will be more resources for straight men to have more children - so they probably will.
Also, this isn't what I meant:
Saying it is caused by multiple genes...
I mean this possibility:
Gene A alone helps fitness by doing X.
Gene B alone helps fitness by doing Y.
Having either A or B helps significantly, so both stay in the population.
Having both A and B happens to make the person homosexual.
In this case, homosexuality would be a by-product of two useful genes. The same could apply for any collection of genes - all that's needed is for each gene contributing to homosexuality to be useful in some other common combinations.
Now if homosexuality did reduce individual fitness (probably true in the modern USA), then the likelihood is that gene combinations such as A and B above would develop mechanisms to shut one-another off - so as to prevent the loss of fitness.
However, since it's quite possible that up until recently homosexuality hasn't reduced fitness to any great extent, there might have been no great pressure for A and B to develop such mutual exclusivity.
and a sort of "continuum" of sexuality would make as much sense a continuum for eating regular food vs gravel or birds flying north vs south in the winter
...or a continuum for liking chocolate, or orange juice, or lemonade...
Such a possibility is utterly ludicrous of course - either people like chocolate or they don't. Such a preference couldn't possibly be a grey area. That would be silly.
Of course society probably puts as much pressure on people to make a clear choice one way or the other on chocolate or lemonade, as on sexual preference. There's no reason to think anyone in a sexual grey area would want to go one way or the other (they don't for chocolate after all). It's not like it'd make their lives easier.
nor would it explain why the concordance for identical twins is not much higher than for fraternal twins.
You know that's actually useful evidence (so long as it's not misused).
Personally I think it's pretty silly to assume there's any single, simple cause for any characteristic of a person - or even that it makes sense to lump all people with that characteristic together.
I'd imagine that homosexuality has many complicated combinations of factors involved. Supposing that it's caused by one virus is about as silly as supposing it's caused by one gene.
In all probability it's no easier (and no more useful) than deciding the causes of liking lemonade.
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