Tony Stark has had many meaningful relationships in his life. Most of them have involved sex.
Tiberius Stone sat next to him in class for all three years of high school (seats were arranged alphabetically), and spent those three years constantly trying to beat Tony's math scores. He never did, and made up for it by kicking Tony's ass in track, squash, tennis, and every other yuppie sport Excelsior prep school forced its student body to participate in. Loser had to give the winner a blow job; since Tony had skipped two grades, he was three inches shorter, twenty pounds lighter, and got very, very good at blow jobs. When they met back up their final year of college, Tony had grown enough to compensate for the two years' difference in their ages. Ty was not quite so skilled at blow jobs -- he used his teeth too much -- but the night after Tony's father died, he let Tony cry on his shoulder for hours, and never mentioned it to anyone. It wasn't until much later that Tony learned that Ty had had his own parents murdered.
He met Sunset Bain in college, too, when he was two months past his seventeenth birthday. She was twenty-two then, older and sophisticated, and wore European swimsuits that were little more than three patches of cloth. She taught him how to make love to a woman, introduced him to good alcohol, and used him to steal secrets from his father's company.
Joanna... sex was really all he had with her, as they both found out after he came back from that trip to Viet Nam. It's hard to keep coming up with convincing excuses to avoid sleeping with someone when you can't give them a real explanation, and women tend not to appreciate it ("I have this metal chestplate I can never take off," is, for the record, not a convincing excuse, even when it's true, and will result in your engagement ring being thrown back in your face).
He never actually slept with Janice Cord, and has always wondered if things would have gone differently if he had. If he'd made his interest more obvious, she might never have gone to Alex Nevsky, never gotten dragged into the middle of Nevsky's feud with the KGB, and Titanium Man might not have killed her.
Whitney only slept with him in the dark, and only with her mask on, because she never, ever let anyone see her face. He told her over and over that he didn't care, and she said that not everything was about what he wanted, and left the metal mask in place.
Bethany reminded him of Steve: tall, blue-eyed, and able to kick a gun out of a man's hand faster than most people could blink. She let him cry on her shoulder, too, and unlike Ty, never tried to use the vast amounts of potential blackmail material she gained in the process against him.
Indries Moomji doesn't count, because in order for a relationship to be meaningful, the other person has to actually care about you and not be a plant hired by your worst enemy. Tony tells himself that she doesn't count, because that way, he can ignore the things she said when she broke up with him.
Heather doesn't count either, mostly because he can't really remember actually sleeping with her.
Cathy Dare does count, even though that first night with her was supposed to be meaningless, just a casual encounter between business trips. It hadn't been meaningless to her, and she hadn't reacted well to being told to back off. Extremis has erased the bullet scars, but Tony still remembers lying on the floor of his apartment, slowly bleeding out while Cathy ranted at him. He tries not to wonder if what Steve felt was anything like that.
Rumiko... there were times when he had imagined spending the rest of his life with Rumiko. He never suggested it to her, though, because he'd never been sure whether or not she'd actually say yes. He'd never been able to figure out what Rumiko wanted half the time; whether they were just casual lovers or something more, whether she really meant it this time when she said she never wanted to see him again, whether she'd meant to hurt him when she slept with Ty, or truly hadn't known she was cheating on him with his ex-boyfriend. She'd made him laugh, though, something that got harder to do every year.
Like Janice, she would have been better off if she'd never met him.
Maya is the first since Rumiko, the first in almost a year, and with her, there is only sex, because whatever Tony had once that let him care about them all (even Cathy, a little bit) is gone now. Dead, along with Rumiko and Steve (and Janice, and Ty, and Happy...). The scar across her cheekbone reminds him of Whitney, especially since they, too, always make love in the dark.
Maya isn't meaningful, but that's all right, because he knows she's going to betray him eventually (they all do, even Rumiko), and until she does, the touch of her hand on his skin and the taste of her lips is the closest thing to love he can feel these days.