I've had an encounter some times ago with a theist over the case of Homosexuality. The discussion could be found in ITB's internal forum (which only accessible within campus).
As a liberal secularist, i immediately challenge the idea that Homosexual acts is morally wrong, even by letting aside religion.
His reason was: any part of our body used out of its functionality (like anus used to do anal sex) is wrong.
AS a rebuttal, i argued that 'functionality' is mean-dependent. Mouth is not for oral sex for sure. But anyone has problem with oral sex? As well as hand with 'handjob'. Nobody got a problem with that.
The debate went on and pretty much equal until the question: What about incest? appeared.
Yes, we all know that is plain wrong, we dont waste time arguing over it. But it seems i couldnt find logical reason behind it.
Then i send an email to PZ Myers about that question. But no reply.
For the record, people in relatively secular like USA, dont have problem with gay.
But if we could not provide a logic reason to ban incest, why do we need one to ban homosexuality?
Later that i know, that PZ Myers actually propose the possibility that science can not answer moral questions, contrary to Sam Harris 'its matter of time science will answer'.
To say it so, its been hundred years since the development of modern science scientist could not find the answer, they let me down.
It now seems reasonable to me that homosexual is wrong. Without any further reasoning.
