I don't know that I've ever said that the supporting cast for Lena is there for sex, but it's expressed in that way a lot. Lena can fuck Louise and Ivy but not romance either. Initially Stan was a perversion choice, peeping, trenchcoat and fedora, wanking it to her pics, generally perving on her. He was reworked later but his romance starts really late and his content can ber otherwise pervy still, even if it's toned down a little bit.
Similarly I don't know that I'd say Ian's cast is there for romance, but unlike Lena, Ian at least have those options. He can start the Holly thing somewhat early and he has more options to "court" several women, he's allowed to actually be interested in them.
The main outlier to both of them is each other of course and like you said Ian can choose to not romance anyone and just fuck everyone too. But at least he has the options. Lena only gets an alternate romance option in chapter 13, though her relationship with Holly of course builds up to it for a while.
The main difference I tend to point out though is the inherent disadvantage Lena has from having such a tiny initial roster of friends and acquaintances. At the game's start it is just Louise, Ivy, Stan and Robert really. Ian by comparison has Perry, Wade, Jeremy, Alison, Cindy, Emma, Holly and Minerva, all of whom are love interests or the partner/potential partner of his love interests. Note that I count Holly in his cast as he knows her before the story begins, though it is of course fair to point out that Lena begins to get to know her too literally right away.
I think Lena should have had another initial friend or two. Perhaps someone she shared her musical interest with, maybe a college friend she could have in common with Louise, which could also help out some of her material since she quickly becomes forgotten if you don't fuck her or Jeremy. The Louise/Ivy rivalry hasn't really been relevant since the bottle-game either so it's not like it would have changed a ton there.