Pegging. Don't even try to pretend you're not curious.
Companions of pegging
-- Anon's guide to which companions are into it. --
Barbara: She's awfully uptight for something that kinky, but she's also very much the dominant type. She wouldn't realize it unless she was introduced to it by a lesbian lover, I think.
Ian: Oh hell yeah, he's got them itchy knees that submissives get.
Vicki: Fuck yeah, she's a kinky street wise party girl. She's more of a submissive switch than Susan, though, and probably only pegs girls who demand her to, or boys when she's ordered to by another woman. Feels like a dom, though an actual BDSM person might do a better diagnosis.
Katarina: Doesn't even know what a dildo is. Embarrasses the Doctor when she asks about it. Definitely submissive.
Dodo: She's a sweet little innocent, so maybe not. Then again, she's very vivacious, so maybe she would. And her name sounds like "Dildo."
Ben: Gets pegged by Polly every night
Polly: Oh baby yes!
Victoria: Oh no!
Jamie: Guy goes around in a dress, acts faux macho, and hugs men with startling regularity? Jamie's totally into it. And that's good for him because...
The Brigadier: Despite Sarah Jane's suggestion that he's a swinger, I just don't see it in the Brig. Too pompous, too serious, too uptight. Besides, what woman would want to go after that butt when she could enjoy the finest mustache ride in the British empire?
Liz: Again, intelligent woman, and from the swinging 70s too. The generation that invented pegging!
Sgt. Benton: A good soldier always follows orders, no matter what form his superior takes, amirite?
Jo: More of a submissive, I think, but she's the kinky type. She'd definitely love riding a woman's cock, regardless of whether she wants to wear one herself.
Sarah Jane: I don't think so, she's very prim and proper and uptight when it comes to that sorta thing.
Harry Sullivan: He's probably tried it at a brothel while on leave, but then he's probably tried everything at a brothel on leave. It doesn't seem like something he'd go for in his everyday life.
Leela: She wouldn't at first, but once she discovered it and got over her initial "You so-called civilized people are so decadent" disgust of it, and tried out, say, a feeldoe? Oh boy, there'd be no stopping her. She'd eyeball a fine young man and decide that nothing was going to stand between her and his ass. She'd be like the gym teacher from Secret Plot Deep, pinning some boyslut down and growling instructions to him about how to please her.
K9: Well, he does wear a collar, and he does call the ladies "mistress"...
Romana 1: Hey, she's a college girl, she's probably tried everything. She's kinda uptight, though. I'm not sure it would be something she'd be into regularly...
Adric: Practically the poster boy for pegging. He probably only acts like the complete little worm that he does towards Nyssa and Tegan so they'll "punish" him every night on the TARDIS. Remember that scene in Castrovalva? Yeah. That, only without all the clothing. Adric, spit-roasted on ladyboners.
Nyssa: She's definitely one of those girls who acts all serious, and proper, and uptight because she's a complete freak in the sack. Probably on Traken they've got freaky bio-mechanical strapons that allow complete 1-1 sensation transfer. And lucky her, she's got the prettiest Doctor and the prettiest male companion to make extensive use of.
Tegan: A textbook example of the kind of woman who thinks all that stuff is only for perverts and lunatics until someone finally convinces her to try it out, and she becomes totally addicted. She definitely prefers getting pegged to pegging, though.
Kamelion: Seems like a perfect pegging potential as his shapeshifting would allow for all potential variations and he does spend time being a sub to a literal Genderfluid Genius but his limited screen time likely links to limited sheet time.
Peri: Nah. Maybe she wouldn't mind receiving a bit of strap-on attention, but it's not her kinda thing...
Mel: Honestly, probably yeah. She'd only do it with guys who asked her too, though. Not the type to bring her own strapon and ask if a guy wanted a go.
Ace: She probably would think it's too weird and kinky until she tried it. The rush of power alone would get her juices flowing, but a feeldoe? Yeah, she'd get real interested real fast.
Grace: Probably Brian would have asked her to try it out and she'd pretend that she didn't like it, and she'd complain afterwards to all her girlfriends (and, if she gets the opportunity, anybody who knows Brian or his parents) that "that pervert wanted me to ass-fuck him with a dildo!" but secretly, she loves it, and she wants to do it to the Doctor first chance she gets.
Rose: She thinks it's something only upper class perverts do, like other weird bondage games. Still, she might like the power rush she gets when she tries it out. She'd only do it with the Doctor, though, never Mickey.
Mickey: Again, thinks it's something weird that rich people do, but secretly he loves it and wishes Rose would do it for him. The real reason he stayed behind in the other universe is he found a hot rich dominant chick who likes to peg him, and she dumped him shortly before The Stolen Earth, hence why he was so eager to return.
Astrid: Nah, I don't think so. She's too submissive. But she might like receiving...
Donna: She acts like she hates it and will gladly complain about guys who like it and try to hold it over their heads to get what she wants out of them, but secretly she loves the power, she gets off just at the sight of a guy bouncing and squirming underneath her.
Jackson Lake: Oddly enough, I think it's his sort of thing, though he'd never get the chance to try it. He probably is satisfied with Rosita's tongue and fingers, though. She looks like she's very experienced at that sorta thing.
Lady Christina: She steals in order to fund her massive collection of exotic, rare, handmade, or antique sex toys. She prefers fucking women with them, but every so often a man might catch her fancy, or at least she might be trying to get something out of him.
Adelaide Brooke: In her younger days, definitely. Now, however, she dislikes everyone she works with too much to think about something as indulgent and pleasing as pegging. She still uses a dildo, she just uses it on herself now.
Wilf: ... no.
Amy: Just try and stop her. No, really, just try. She'll get you drunk, high, way turned on, really guilty, whatever it takes to pry your legs open and introduce you to her tardis blue strapon. "So many games of Doctors and Nurses" pretty much speaks for itself, especially when you remember her husband's the nurse...
Osgood: Who do you think K-Stew's pegging all the time?
Vastra: She's well versed in strapons, Silurians having developed the technology millions of years ago, but she's not a big fan of men, as we all know. That said, she's also not the best judge of gender, so a feminine guy who caught her fancy might get to know that super-tongue really well...
Strax: As far as Strax knows, that's how earthlings make babies. Nobody's going near his probic vent with one of those things!
Clara: Bossy control freak in the streets, bossy control freak in the sheets.
Handles: It's his only method of interacting meaningfully with other people. Remember when the Doctor stuck him on that big stick in the TARDIS? It's just like that.
Ashildr/Me: In an infinite lifespan, you can become the best in the world at any skill you decide to take up, especially buttsex. Gotta avoid having kids somehow.
Door: Opens to anyone a little bit nice, very clearly a sub. While there may seem to be the issue of were exactly you'd stick it, the answer to that is obvious: the keyhole.
Nardole: Why do you think River kept him around so long? She needed to keep busy while Hydroflax was busy dying.
Bill: She's a dyke so it speaks for itself.