So...

I mean when youre first dating or going out on dates with someone its very hard to ask "what are your secrets?" Especially if the person doesnt seem that interested. Its not like they were instantly married.

It's not that he think the seme is the bad guy, just that who would be patient enough to date a guy stop his merely 30 minute date for everytime his alarm goes off. Maybe the break up was only him talking in the heat of the moment (like how some would often do in real life too) but the seme didn't make any efforts to explain.

Well, I don't think it necessarily means asking the other party "What's your secret?" since the timer publicly rings, but other people usually ask certain things especially when it occurs more than just a few times. Tho I guess I can't say for other countries.

I don't hate the MC, but I do wish he asked first what the timer was for since it is bothering him so much, before announcing a break-up in the middle of dinner. If the other party doesn't want to answer that then fine, that's their secret. This is not only necessary in marriage but in most of types of relationships as well, so I'm very confused as to why you ended your statement with them being not married.

Yes, the seme is also at fault for not telling the MC about why since they've started or at the least while they were dating. I understand that a lot of people, myself included, will be triggered by it. Like what is this, I get an amusement park token for a day and that'll be all once the timer stops? Tho I would end up asking, and will break it off if the other party doesn't even seem interested in letting me understand why it's happening as it is and just lets the relationship break off piece by piece.

In regard to your second sentence tho, I dunno. There's too little context to even know what happened after he said to break up, heat of the moment or not. It just literally ended there ╮(￣▽￣)╭