I gave an example where the person would exactly have PTSD.
The thing is I know someone who has PTSD from being raped and its not like how you describe. All of their triggers they've described to me are things pretty directly related to the rape itself; either to details of the attacker or more generally to the concept of non-consensual sex. And yes, they actually have been triggered by an internet post. Not in the ironic making fun of SJWs sense, like actually having a serious reaction.
Now you are right that sometimes triggers can be things that are not entirely obvious, and its true that the more intangible or harmless triggers cannot be worked around. But the obvious ones can be worked around. Like don't show people graphic depictions (written, audible or visual) of rape with no warning, because it actually can effect people. Someone who is triggered by gay kissing, well I mean they'd be an internet troll, but if that was a thing for some reason than asking for warnings there would not be a reasonable request.
Your example is in poor taste. Rape is not in the same category as normal romance or sex. Nobody is asking anyone to take down or label images of kissing. Indeed trigger warnings are not supposed to apply even to porn unless that porn simulates rape in some way.