That actually wouldn't weird me out that much - she's pretty old for her age, and nineteen is old enough to take that kind of commitment seriously - if the text weren't so fond of making an issue of it. W&P kind of go out of their way to remind us that Donna's a rebound off Terry's first marriage and her whole social circle is too young for his whole social circle at every possible opportunity. If it really didn't matter, it wouldn't usually come up, y'know?