I think this depends on the grade level. I don't think they should be forced to remove it, but they shouldn't be explicitly promoting it if that makes sense. But I'm talking more about the pride flags and posters rather than a discrete photograph. A lot of people tend to make being LGBT their main identity and that shouldn't be pushed in a classroom setting.
What does "promoting" it mean? Your criticisms here don't really seem any different from the "stop shoving it down my throat" whining from the 2010s whenever a gay couple appeared in a movie or a television show.
It’s not homophobia if you’re keeping it consistent. Banning pride flags isn’t homophobic if you ban straight pride flags as well. It’s not homophobic to prohibit gay teachers from mentioning their spouses if you ban straight teachers from mentioning theirs.
This is different from it being a good idea to ban to those things, however.
What is a straight pride flag?
And nobody is, in practice, going to ban a straight teacher from mentioning their spouse.