Seems moondog is far from the only leftist who use "casual homophobia" to attack Trump and other Republicans. It's common enough for the usually execrable HuffPost to write an article about it.
HuffPo has written several articles about it, as have several other "leftist" publications. This issue is not restricted to the left. I think the reason it is getting so much recent coverage is that there seems to be a misguided belief among some that since Trump is such an awful excuse for a human being, homophobic jokes about him are okay.
No one who uses the term would disagree with you that casual homophobia is deeply hypocritical. The casual part of the phrase just emphasizes that you don't have to actively twirl your mustaches and decide to hurt homosexuals to be homophobic. Buying into inherent anti-LGBTQ assumptions embedded in language and society is enough to hurt those with different sexuality, and is thus homophobic.