Sex in a relationship is a difficult thing. If it's too soon (there isn't an exact time, because everyone and every relationship is different) but if it's too soon it will likely turn in to ALL the relationship is about, and will end. If, for some people, the wait is too long they will feel sexually frustrated and let their sex drive control them, therefore ruining the relationship because of sex. Two people need to be able to openly talk about sex and what they want in their relationship before they have sex. It's hard to know for sure if you're ready, but if you can talk [seriously] about it and your relationship with someone, then it's likely that sex won't ruin your relationship.