Unless you're on an nagpur call girls themed site, it's unlikely you'll get anywhere by messaging women and straight up asking them for sex. Of course, there are exceptions, for example, when a woman has indicated on her profile that she is also looking for sex. Otherwise, it's better to be more polite. Depending on the site you use, it may be acceptable to state in your profile that you are looking for sex, although different sites will have different tolerance levels for such requests. On normal websites, it is best to keep such information to a minimum without resorting to any graphical descriptions.

Women probably won't bother contacting you if your profile doesn't have any information about you. It pays to get it right because the number of responses you get will increase significantly.
When you start interacting with women online, don't act like you're going to get married right away. Wanting to know where your new friend is every time she doesn't talk to you probably won't make her stick around.

It's somehow hard to tell the difference between coldness and distance, and the difference between kindness and over-eagerness. Still, sometimes it's best to err on the side of caution and not overwhelm your new match with over-the-top romantic behavior.  If you like someone, let her know, but again, don't expect her to commit to marrying you after just one phone call or date.