To me I think a relationship is serious when you have breakfast with the person. This does not really include breakfast at home. Here is what I mean. Yes, it is a big deal if someone spends the night after you had sex, but it could also mean they were too drunk to drive home or they have six roommates who all snore and they are really enjoying the fact they are only having to share a place with one other person. It could also mean you have more options on your cable. The true sign of something getting serious is when you go out for breakfast after spending the night. To me this indicates that not only did the two of you spend the night together but you also liked each other enough where you are spending the morning after on a date and going out to breakfast also probably means you are going to spend further time with each other that day. So, the photo of Scarlett Johansson and her new guy having breakfast together shows signs of seriousness. Of course with her last boyfriend she went on vacation and chain smoked and broke up two seconds after they got back.