On top of all that America also just actively persecutes queer people. People will say "oh that's just the republicans" as if that isn't half the country and it's not just the republicans, but a large group of democrats. Then they'll say "oh, but we're not as bad as [enemy of the week]" which funnily enough is actually a whatabouterismerino, but also not really a valid argument if you're saying it's okay to bomb a country if they're mean to queer people. So it's okay to be kinda mean to queer people?
On top of that there's also the fact that some of the US' closest allies are countries like Saudi Arabia, not to mention the many far right anti-lgbtq dictators the state has installed over the years.
On top of that the US has only recently gotten "good" on queer rights. Homosexuality wasn't decriminalised in the whole of the US until 2003 (and we're not talking some weird little forgotten law, it wasn't until a supreme court decision forced several states to finally stop being bigoted.)
At the same time the GDR had decriminalised homosexuality by 1957 and with constitutional reform fully legal by 1968 
The GDR did this despite inheriting the nazis legal code (according to a wikipedia source which I will not fact check.)
Would it have been acceptable, nay morally right, for the GDR to bomb the US?
definitely, but not just because of da gays
For me I've had some succes with responding with "So you think it's okay to kill them because they're bigots?"
Sometimes they do a bit of handwringing, sometimes they don't, but they end up arriving at the point that yes they think it's fine that they die because they're bigots, we need to make them more accepting of LGBTQ people.
"Alright then, we should kill all the bigots in this country. Let's go murder all republicans. Let's arm trans people. The death penalty is legal, so let's just make it carte blanche."
Then they'll get mad and talk about false equivalencies, but you've basically got them at this point.