Sorry, man, I didn't know you would be so offended. You can meddle in people's conversation here, no problem.
Are we supposed to believe that a meteor will fall in the middle of Tokyo and make everybody aliens or something?
That's an example that shows that by your argument, everything is dumb. We are talking about fiction here, the enjoyable thing about fiction is that we can imagine things that will never happen. Almost every romance is not believable, it is all very overdone in many points, you know that it will never happen in reality, but that's what makes it enjoyable. "What if a couple who makes a promise in their childhood meets again...", I could go on and on with the examples.
The author's purpose is not to make it believable, but interesting, surprising, chocking, etc...
Every joke is a truth on the background. I really feel that people here don't like romance that much, it is not everybody, but I put a little of exaggeration for the sake of the joke.
It is funny you mentioning Suspension of disbelief at the same post that you negated it to harem mangas. What I just described above IS suspension of disbelief.