As Eva talked about in the recent Q&A, Ian's and Lena's paths aren't meant to be symmetrical, so it's true that Lena can get away with some things Ian can't. But it's not completely true that she can sleep with everyone without consequences either, just like it's not completely true that Ian always gets punished for it.
A big part of the reason Ian can't sleep with everyone in the same playthrough, is that he's got a big group of friends (which Lena doesn't), and most of his sexual options are within that group. So he'll have to completely or partially choose between Alison/Cherry, Alison/Emma, Holly/Cindy, Holly/Lena and Lena/Cherry. When he meets someone outside of his inner circle, like Minerva, he can do what he wants without consequences, just like Lena. He's also a guy who's both fairly attractive and have some personal characteristics that make him the kind of person several of the characters could fall for romantically. That's an advantage in some scenarios, but it's also a disadvantage if you want him to sleep around as much as possible.
But there's also limitations on Lena's path: She's got to completely or partially choose between Mike/Louise, Louise/Jeremy, Mike/Robert/Ian and Ian/Holly. Louise, Jeremy and Holly are her only options within her or Ian's group of friends. Louise is an insanely jealous person, and that's bound to cause trouble; she's also likely to be in deep trouble with Louise and Ian if it's revealed that she's involved with Jeremy; and Holly is only an option for her if Ian doesn't take his chances. So when the conditions are similar, Lena's also got similar limilations as Ian.
And while Lena can sleep around with less consequences, there's other areas where her conditions are way more difficult than Ian's. Even though Ian doesn't like his current job that much, it's still within the industry he wants to work in, and he's got very few financial troubles. He can keep his job with or without sleeping with his boss, and if he somehow manages to use all his money, he can still ask his dad for a higher grant and/or Perry to delay payment of the rent. Lena had to quit her studies to make money for her family, and she also ends up losing her job even if she sleeps with Robert to prevent it. And the only way to progress her modelling career (at the moment) is to do more daring shoots with Axel and Seymour. The continued presence of her cheating ex-boyfriend also makes it harder for her to move on than it is for Ian. Also her two closest friends from the start of the game, Louise and Ivy, can't stay in the same room without making life miserable for everyone around.
Whether Ian's or Lena's limitations are the most frustrating is down to how each player wants to play the game and shape the characters, I guess. But personally I'm more bothered about Lena's struggles with her working life and social life than the fact that Ian can't sleep with everyone in the same playthrough. And while it's also true that Lena doesn't experience the kind of slut shaming women often get if they have many sexual partners, she's the victim of another realistic gender related consequence of the situation she's in: the many dudes (Seymour, Axel, Billy, guys on Stalkfap) trying to take advantage of her in the vulnerable situation she's in.
