I'll give you a debate if you want, even if the views I express are not mine -
Firstly, you assume that certain acts are only banned (or should be banned) because they cause harm. However, this is demonstrably not true. Binge drinking, binge eating and boxing all involve activities which may cause some harm, yet they are not banned.
However, some acts which do not cause harm are banned, such as marijuana, solicitation, or bringing Cubans into America.
Therefore, it is a fallacy to generalize and say that certain acts are only banned for they will cause harm which is not true.
In regards to why some countries (mainly Islamic) disapprove of homosexuality, they feel such relationships serve no purpose other than to fulfill carnal pleasures, a situation they find deploring because everyone should be concentrating on accumulating enough good deeds to go to Heaven. Whilst satisfying one's pleasures may also be factor in heterosexual relationships, the fact that it may lead to something greater (i.e: a child) is something that Islam considers satisfactory for the engagement of such a relationship.