Because one doesn't necessarily follow the other - having very little morals about one thing doesn't mean this applies to everything else.
To take an example: a person who burgles houses regularly could otherwise be a very caring parent - despite having no morals about stealing from others it doesn't necessarily follow that they have no morals regarding their children.
And likewise although Garak has clearly engaged in murder and deceit in service to himself or Cardassia etc it doesn't necessarily follow that that dubious moral compass also applies to pursuing a relationship with someone who is, what, thirty or forty years younger?
Garak may well have been gay, I'm just refuting your opinion that him having dubious morals in one aspect of his life automatically carries through to all other aspects.