Very late reply, but I believe that Star and Tom got together too fast in terms of viewer's point of view. For what we can expect, they probably had many times together offscreen and the episodes are just glimpses of what occurred. The same with Star and Marco. They spend many months together, yet we don't see ALL interactions.
To put it in perspective, let's just say Star Comes to Earth takes place in January, just in time to be enrolled to school. By the second season, it's summer. Those are many months with Marco. And then after returning to Mewni, we can assume that after the dance, some months have passed after the fateful dance and that Marco stayed on Earth for much longer than we are lead to believe. Then Stump Day, which likely takes place in December. We have the two main characters already having birthdays and a year worth of interactions we never saw.