After successful seduction and sex, you can be calm, because in fact you achieved what you wanted and became very close to your lady. If you like a girl and you want to continue dating her or have a romantic relationship, then be yourself and you should not invent special vulgar phrases. They can be perceived by a girl both humorously and negatively. So just show concern. Find out how she is doing, if she feels good. The main thing is to make it clear that she is important to you and it was not sex for one night. By writing first, she will appreciate it and will understand that she is important to you and you are not one of those guys who will immediately look for a new girl for sex, but really want to continue a relationship with her.
However, there may be another situation when you realize that the girl is not very attractive to you and you do not want to continue communicating. Then just relax and do not impose on the girl. You can follow the position of a courteous guy who knows how she is doing, and then just take the time saying that you are busy and you have a lot of work. In a short period of time, a girl is likely to meet her chosen one, with whom she will be happy.
In the event that you want sex without obligation, then it is better to immediately say about your intentions and unwillingness to enter into a relationship. This is a medal with two sides. Either you get what you want, or you will be refused.
One way or another, what exactly you write to the girl is not so important. The lady had enough time to form an opinion about you, and even more so since it came to sex, then she likes you and you should discard all prejudices about yourself and not worry about what exactly you will write, because for every girl the most important thing is - attention.