Okay. A corrections.
Teen and adult members can definitely imply that their characters have sex. Reason being, that isn't actually a sex scene. It's the difference between writing romance and writing a sex scene. Say two characters have had dinner together, they go home together, one leads the other towards the bedroom and then you skip to the next morning with clothes all over the floor, no one is going to get in trouble. That is not a sex scene. Both writers and readers know that sex happened, but nothing about sex was ever written. It is not a sex scene.
Foreplay starts pushing the boundaries. The hard hand fast rule, as much as there is one in the gray fuzzy area between romance and sex, is that as soon as anything relating to genitalia gets mentioned, it's gone too far. Therefore, making out? Sure. Kissing is just kissing. People do it on the street, no one gets pissy, the law doesn't care. Groping? That starts crossing the line. As soon as a guy's "bulge" or any other such words are mentioned, it has definitely gone too far. Any full description of naked bodies or bodily functions, male or female, is too far.
In general, it is best to just play it safe. Why risk it? There are plenty of members you can legally do such things with. However, roleplay characters are totally allowed to have sex as long as no sexual acts are ever detailed in the writing, no matter the age groups involved.
If you need some more definitive examples of what is an is not crossing the line, what's really pushing it, etc. I may have some old examples stored away somewhere on my hard drive I could pull out. But another good rule of thumb is, if you are starting to wonder if you've taken it too far, it's probably gone too far. :D