I actually like traps ..So seeing people get pissed when they realise its a guy, is funny to me.
A guy?! kill it!
It's more about raising expectations than traps. Part of the issue is that japan still tends to consider that female homosexuality is just a phase that women goes through before settling in and marrying. And a "popular" form of fanservice in many anime/manga is playing the yuri subtext more or less obviously.
Combine the two and you run in anime/mangas that raises up your expectation of yuri happening before crushing it with some het. That made me wary of a lot of titles even when it has subtext and still some male character around, so yeah, I usually feel "safer" when there's no male character around because that usually reduce the risk of a bait & switch considerably.
Here, we're introduced to a character that is basically a girl design-wise, after the girls were chatting about who they liked, said "girl" started kissing and flirting with the other girls all-around the chapters, so yeah, it looked like a yuri one. And then, it's suddenly "oh hey, by the way, I got a dick !"
I read the spoilers beforehand, so I was prepared to it, but I still can understand why people who avoided the comments or spoilers before those chapters got annoyed at this turn of event.