These are some of the areas i can think of if you want to argue on a secular basis:
For
- rights of homosexuals to a good life
- freedom of choice
Against
[/LIST]A little more thought can turn out more arguments, both for and against.
Unfortunately people's decisions are based on beliefs, not logic. As such, the best that can be hoped for is to give enough evidence for further investigations and discussions. Hopefully for the individual, sufficient to make a more reasoned judgment.