السلام عليكم,
I've had many arguments with non-Muslims about Islam, and they always bring up the homosexuality topic and its punishment and view in Islam; As I'm just a common person with no deep knowledge of Islam, I usually answer them by pointing out that I'm not entitled to justify Allah's motivations.
The issue here is that I have to come up with a purely logical reason to convince them that homosexuality is wrong in the modern societies(as they don't believe in Islam, nor god). However, after doing a small research, the only applicable argument I came up with is Ahmad Al Deedat's speach on the topic, although it was I need more convenient reasons and arguments to assist me in proving my point.