It's true this pattern of "Accept <wrong> to prevent <worse>" has nothing to do with left or right. However I don't think it applies to pro-gay propaganda, because the whole point of the left-wing stance on this issue is that there is nothing wrong with being gay in the first place.
Side note: the pattern "accept <wrong> to prevent <worse>" is of course not bad in itself, but rather when false assumptions are used to connect <wrong> and <worse>...
this is a basic principle of all propaganda, right wing, left wing, anti gay, pro gay, white suprematism, black power, the list goes on.
Let's give these banks just a little trillion dollars or our country would suffer. Let's burn just a few Jews in the furnace or they would sell our country. Let's take all the money from the rich and kill them and make everyone equal(ly poor). Let's bomb these Muslims into the Stone Age because they threat our children. Let's help those Muslims kill Serbs or World Democracy is in danger. Let's allow these morons squeeze our balls and tits or our airplanes would fall from the skies.
It's just marketing, a way to sell the bad idea to people and make them feel happy. It has nothing to do with right and left.