"Have homosexuality"? - Did I say that? They all engage in homosexual behaviour - or do you suppose that two female macaques mounting each other and rubbing their genitals together or two male monkeys giving each other hand jobs is incorrectly classified as "homosexual behavoiur"? Of course animals don't generally choose a homosexual "life style" - its not part of their culture. Only humans and sheep are known to limit their sexual activities in that way. Most other animals, it seems, are quite happy to get what they can either way with what would seem at face value to be an obvious naturally selective advantage for those who swing more towards heterosexuality. But given that we each have a bit of both male and female genetics in us, it could be that a genetic trait that promotes female fecundity might also prompt males to be more attracted to other males if the genetic balance is weighted in that direction. That's just pure Darwinism - Darwin wondered why animals produce far more offspring than seems necessary - which is just another way of asking why females are so fertile. The answer, of course, is that this 'over-productiveness' ultimately allows nature to weed out the weaker genes and ensure the survival of the species. But it seems like the price of this enhanced female fruitfulness is a significant increase in male fruitiness in a significant proportion of the populations. The first (obviously advantageous) effect outweighs the the second (seemingly disadvantageous) effect.
Point being - the topic asked what science says about homosexuality. My answer is, it is almost certainly genetic and it is widespread in nature. And BTW - I never said that hetero- sex was just about reproduction - sex, in any form, for any species, is about much more than just reproduction - its about bonding, its about social status, its about fun, pleasure, aggression, submission, control, domination, giving, forgiving, passion, compassion, empathy, selfishness...its hormonally very complicated. Its probably the most complicated and least understood part of being alive in many ways. Which is why I find the idea of a 20-year old male and a 28-year old female presuming to teach each other about it in a discussion forum quite amusing. I've got nearly double their years on my clock and I still don't understand it - but I reckon understanding it is not the point.
And you call my argument odd? How is consensual homosexual sex even remotely like the third-party suffering inflicted by eating live animals or destroying the environment?