Taliban stop female Afghan students leaving country to study in Dubai

[Angrboda]

Saying gay sex is wrong harms gay people (and thus society) who might grow up in an Islamic household and have inculcated within them an undue sense of shame or guilt.  They may try to change or affect their sexuality or behaviour to fit in with this teaching.  It also causes offence without justifying reason.

Additionally, by saying gay sex is wrong in an authoritative text or within an authoritative institution makes it far more likely that people will act on that belief in all manner of harmful ways, from voting for politicians who run on a platform of bigotry and try to enact discriminatory laws all the way to Islamic societies throwing gays off rooftops.  Thus saying gay sex is wrong increases the likelihood of increased suffering in the same sort of way saying smoking is good for you or Jews are evil does.  

It is reprehensible speech for all these reasons.

Those facts are only incidentally related and not necessarily related. You can't say John is evil because he might rape a teenager. Only that John is evil if he has or will rape a teenager. And likewise, someone raised in a Christian household that embraces hate the sin and not the sinner, may grow up feeling loved and experience no shame or guilt or regret about being gay. And beyond that, this all depends upon feeling those things about being gay is wrong, which you've simply assumed. Just as thinking that society would benefit if religion was eliminated is only moral if it is actually true, if gay sex actually is wrong, there is nothing immoral about stating that fact.