I am half-remembering that there is another question on this site has brought up the concept of lines and veils to handle such situations. Simply put, lines are topics that will make a player leave the game permanently if you do it (cross the line). Veils are something that a player is comfortable enough that it would happen in character, but really does not want to roleplay that particular scene.
Torture is frequently a veil topic. What I mean is that the player gets captured by the bad guy who then says "tell me all you know about the Macguffin". The player can have the token defense of "I'll never tell you a thing!" Then the DM says something like "OK, Count Evil Guy starts torturing you. How long do you hold out until you spill the beans?" Sooner or later, the character will say SOMETHING just to make the pain stop, and with Truth spells, lies will be detected pretty quickly and likely punished severely. If the PC decides he won't talk, then simply the DM should let him know that the bad guy is willing to beat him to death to find out. Sure, it breaks the fourth wall, but gives a player a chance to save his character before the DM feeds said character to the shredder. If the player believes the character won't spill, and the bad guy believes the character will, you can "veil" the torture as either "Sir Brave dies a horrible death" or "Sir Brave suffers torture until he spills his guts, takes [player decided number of hitpoints] damage", and the player can describe the wounds any way s/he wishes.
Rape is frequently a line topic. Seriously, if you are an all male group, and this character is female (or ANY of your players are female) don't do rape without the express written consent of any females in your group. I've gamed with many female players over the years and they all have a story of "I thought this group was great, then in the second/third session, my character was raped (sometimes repeatedly). I left and never called them again/returned their calls." The topic is something that is not okay. It will never be okay, and people who think this should be ashamed of themselves. One of my personal lines is Chicago in Shadowrun. The fiction of Shadowrun has the city of Chicago overrun with insect spirits/shamans (or at least old versions did). I do not want to run in that. I don't mind the occasional bug-hunt, but have no interest in playing in that environment. The one time it happened I told my GM that my PC was committing suicide in right after the Johnson mentioned we would have to go into Chicago.
How do you find the veils/lines? Drop your players a quick email individually (don't just write up the email and CC to everyone else... you get maybe one response and a bunch of "me too" answers), describe the concept of lines and veils and then ask them how they feel about their characters suffering any of the adult themes you have in mind. I would ask about more than you currently plan, just to keep them guessing. A good start would include torture, rape, PC having sex with a "throwaway" NPC (prostitution/casual sex), PC having sex with a standard or recurring NPC (relationship), PC having sex with another PC (however the players choose to play it), aberrant filth (squalid prisons without privies), PCs being anti-heroes or outright villains. Ask them how they rate each individual adult theme on a scale of "Line", "Veil", or "Fair game", then take the most conservative answer from the players. It also helps to ask them to differentiate their answers between "other PCs" and "my PC". I have no problem if someone wants to act out a torture scene with the DM... I will likely ask that either the DM or another player punch me when the torture is over, and play some game app to kill time until it's done. However, I would prefer any torture that will happen to my characters be handled as a veil. Another player in my group would cheer on the DM during the torture scene, but would prefer his PC's torture be a veil also.
Found the question here