My boundaries are a bit iffy, depending on context. Depends on who does it, and how it brings the story forward. I can see bestiality if a character is transformed into an animal* and Necrophilia, if one of the persons involved is a vampire who has sex with their food, or maybe a ghost...
But molestation and pedophilia is right out. I have worked with a person who turned out to be a pedophile (yes, he got convicted for it). That was not nice.
* For example in our Scion 2nd ed campaign, my character joined, and took control of the Wild Hunt. One of her titles is "Lady of the Wild Hunt". When she joined, she took the form of a 3 metres tall Wolf, who used an Ogre as a chew-toy. Had she met the Fenris Wolf in that shape, she would have gladly mated with him. Both mine and the GM's idea there was that the Wild Hunt is a primal force, where you will get certain urges. So when she transformed back into human form, she still had lots of adrenaline and those urges. About 50-50 chance of her killing someone she met and drinking the persons blood or screwing their brains out. Yes, she spent 2 days in a hotel room with someone she picked up on the way back... That was what it took to return her to her normal senses. The reaction will be much less next time she hunts, but she took in the full force of the hunt when fighting the ogre...
In the same campaign my character has been the victim of something that technically would be rape (that she was very willing to the deed, but misled to the identity of her partner is a different matter). Loki had taken an interest in her from day one. He then later locked away her husband in Vanaheim (somehow the realm got locked in so no one could enter/leave it, and Oden has spent quite some time trying to open the ways again). Loki then masqueraded as her husband for quite some time (months at least). Let's just say pregnancy happened. And of course there were a prophecy that their children would cause Ragnarok when (not IF) certain conditions were met. We ended last meeting with her giving birth. She has gotten the suggestion from both Frey, and her mother Skadi, and Apollon that she should drown her children... She had planned on an abortion when she found out she was pregnant and that it was Loki, but various time-related effects and a trip to another dimension to save the world precluded that.
edit: At the moment, she is NOT happy with Loki, and seeks ways to punish him. But there is an attraction, and sooner or later she will join him voluntarily (think Buffy + Spike). They are very strongly fatebounded to each others.