In the example of Putin/Trump, it's more like saying Trump is submissive to Putin. The insult being that HE would find that intolerable, not that it is inherently insulting to be sexually submissive. It's the same reasoning behind calling him out of as a draft dodger. You might think the draft was horrific and not blame your art professor for taking a long ski trip to British Columbia, but aggressive wannabe He-men who got out of fighting because they had a sore foot are a different story.
As someone who has sucked a dick or two on my way through life, I think there's a difference between that kind of insult and, say, the careless use of cocksucker. The target is the persons hypocrisy; they aren't living up to their own shitty ideas of manhood. It's not the suggestion that the act is bad or shameful, or that a person who would do that is shameful.
One problem I have with some of the language policing going on with the left is they sometimes treat people like they're all idiots. Like, even if they themselves can distinguish the subtleties of use, the concern is that others won't be able to and it will enflame a problematic correlation. But I think people can see the difference between saying Trump sucks Putin's dick versus Trump is a faggot.