Well, his few words did kind of work to cheer her up... but in this kind of situation, mostly involving something serious like marriage i thouhgt he would at least start with "whats wrong?" instead of gambling about her reasons and saying something so casual. But then again, Oyamada is Oyamada.
Marriage does have its complications. Its something that is suppose to merge both lives, like my life and all in it is also ur life now. But its up for the couple to do whatever they want with it. Most of all, marriage in a more simpler definition is a official commitment to share ur life with that single individual forever.
Osawa fear is completely natural. She wants to protect what she has now, so changes are a huge risk. In their case i dont think it would really change much of what they have now. Either way, i think is too soon to think of marriage and societal standards about her age and all are exerting an unnecessary pressure on her. They had been dating for less than a year and literally just move in together. Just enjoy that before thinking too far ahead.