Author Chapter 3.1 (Jitsu wa Ore, Saikyou Deshita?)
MC wants to leave his home so he can become a shutin?
Yup makes perfect sense.
what?, this does not make any sense, how the f*ck the daughter can't deal with someone of the opposite sex, that guy over there has been living with them for 9 years and still, she never talked to him?, what the fck dude
They were raised as sibling, this makes no sense
Adopted kid.... is the heir...

Younger sister... despite apparently the parents not being able to breed naturally. Maybe they got over it. Maybe she was also adopted. The latter would explain why she wasn't inheriting... or else they're stuck with Agnatic succession...

Dude is... 10 years old... but somehow thinks of himself as a 30-year-old despite never having aged past 20 or so. Arrogant little shit reckons that because he adds his two lifespans together that he has any clue what it is actually like to be 30? Age doesn't work that way, idiot. Age experience is more than just a number.

@Kaarme : That'd defeat the entire purpose of isekai. The genre is about empowerment fantasies for the underdog... not rewarding the already successful with even more success. It rarely ever feels deserved.
@SotiCoto I'd hesitate to agree on that generalisation. I feel like I've seen plenty of isekai with rather normal MCs. Some might have faced difficulties after being summoned/reincarnated, but would have been just fine in their original world. Tate Yuusha is a very stereotypic series where the MC is treated poorly for no reason of his own, but he was nothing weird in his original world, just an ordinary otaku university student. I suppose that made his treatment in the new world worse because he wasn't used to it, like a real underdog. Another classic of the more neutral kind, Slime Datta Ken, has, again, a perfectly ordinary office worker dude. He's pretty well off in the new world. If we take something like Mondaiji-tachi, the male MC is actually more like a bully than anything like an underdog in the original world, and in the new world he's even better off.

I'd need to go through my list of read series to make a real list of examples, so I'll just leave it at that.
@Kaarme : Well okay, there are a few cases of isekai where punishment seems to be the purpose rather than compensation for a poor life... though admittedly only the infamous nazi loli of doom comes to mind right now. Still, I can't think of any isekai protagonist who was genuinely happy and successful in their life to the point where it would make others envious. The most usual situations are loner / otaku / outsider highschool kids... or overworked black company employees.
@SotiCoto I'd say it's more of a problem of the very structure of the story, rather than being an underdog the readers could sympathise with. After all, if you had an MC that was super successful and enviable in their original world, it would inevitably lead to a situation where the MC would want nothing more than to return to their original world. While wanting to return is not a super rare condition in isekai, generally speaking it's not the only motivation, by far, in the head of the MC (or MCs). Unless the isekai is all about finding the way back, it would just get annoying before long.

That being said, for a RL underdog, like a bullied school kid, just a regular fellow is already something to be envious of. I consider the romantic inexperience of the MCs more of a joke than something that would make them underdogs and losers in life. If you are decently good looking, have a solid job, or are a student doing just fine, there's nothing in particular preventing you from having a romantic relationship. It's just the usual riajuu, explode joke and the shounen genre virgin status that keeps them inexperienced. It would also be sad to leave a girlfriend or a boyfriend behind when summoned/reincarnated. Actually, that could lead to an infuriating plot. I was going to mention a series name as an example, but it could be a spoiler if you aren't reading it.
Unless the isekai is all about finding the way back, it would just get annoying before long.

That is more classic isekai than modern isekai. Like Magic Knight Rayearth. Effectively a different subgenre.

I consider the romantic inexperience of the MCs more of a joke than something that would make them underdogs and losers in life

They're generally portrayed as losers for being that way though, and at some level often consider themselves so.
I guess I'm not really a good judge on that sort of thing though since my particular perspective is really abnormal one... so I can't really relate to their situation at all.