Only having sex when you are planned to get Married? That's so weirdly puritanical, I wouldn't call that Culture at all personally. Requiring marriage for sex is based on religious doctrine, and only that. I get being in a relationship first, I totally understand that, but why do you need marriage to be on the table? That's what's so weird to me.
It may have its origins in religion and puritanical culture, but it has become a fairly widespread belief nowadays. Just because you may not agree with it does not mean it's an outlandish point of view.
I could just as well call
you weird for being willing to have sex with someone you wouldn't consider marrying. Who's to say who is in the right?
Not to mention that Makoto's views are probably based around Japanese views of marriage and intimacy (maybe even more on the conservative side, based on his character), which will obviously be different than your (presumably) Western views.