Some *young adult* publishers will accept graphic sex involving teens because the books are FOR teens. It's presumed that teens know what sex is, and that enough are having it that it isn't unreasonable for it to be present in books that teens will read.
However, most, if not all *erotica* publishers (and erotic romance) will not accept graphic sex between characters under 18 because the books are for ADULTS, and there's a very sketchy line with presenting underage sex to an adult audience.
There's also the matter of the intent of the scene; in young adult fiction, sex scenes are supposed to contribute to the plot and/or characterization in some way, and the focus is usually on the point-of-view character's emotions and thoughts. In erotica/erotic romance, sex scenes are supposed to do the above but also are supposed to *arouse* the reader. And the focus is often on, or also on, the physical mechanics of the act and the physical sensations experienced by the POV character.
Most publishers don't want to deal with scenes of teenagers/underage characters when presented in an arousing, titillating way, and especially when presented that way to *adults*. (And yes, adults do also read YA. And some of them probably get turned on by it. But adults are NOT the intended audience for YA.)