A good try but you've got your metaphors mixed, as the "glass" analogy relates to pessimism and optimism. The question is often couched as one of 'nature vs nurture' and usually pits predeterminism against free choice (which I guess could be viewed as 'pessimism vs optimism' crudely, but I digress...)

The answer is really 'both,' to the extent that behaviours can be coded in genes, but genes don't quite have a 'one-to-one' impact, as developmental and environmental factors affect the expression of genes. Cultural, social and experiential influences seem to have far greater effect on sexual orientation.

If you're really interested in the literature on the matter you may want to start here [edit - wikipedia, Biology_and_sexual_orientation]