Ok, comedy lesbians are a type of character that shows up a lot in anime/Visual Novels/Light Novels/games etc (I have seen a few in western works, but I'm gonna focus on the Japanese side). There are several variations on the concept but the key elements are as follows.
First, they are always a secondary or tertiary character, if part of the main cast they will usually get the least amount of focus.
Second, their entire personality revolves around "liking girls". Sometimes this is just any girl who's attractive or cute enough (like Soleil) or sometimes it will be one specific girl that they focus all their energy on.
Third, their feelings aren't reciprocated. This itself isn't an issue, but it is in concert with the next point
Fourth, their feelings aren't treated as being "real" or "serious" by the other characters or the author/writers. They frequently get dismissed as being childish, are told to grow up and get over these silly infatuations, and often explicitly are told they need to find a man.
Fifth, these characters are often over-aggressive to the point of actively molesting other girls (breast groping in the bath, etc) and older versions of this type of character often come off as explicitly predatory and rapey.
Sixth, they never get a girlfriend. The story will either end with their feelings explicitly rejected or they will be paired up with a male character who effectively "cures" them. this is most common in Visual Novels, when the character has a route with the MC and very often he will literally fuck the gay out of her.
Perhaps the most well known of this sort of character is Shirai Kuroko from the "A Certain Magical Index" and "A Certain Scientific Railgun" series. She tics off a lot of these ponits. She's hyper-aggressive and almost rapey to the object of her affection, her feelings aren't treated as being serious by other characters or the writer. Despite her being a powerful teleporter, she's frequently undermined by the story, often using her crush on Mikoto to do so.
Soleil hits almost all of these points. She's actually almost the perfect example of a Comedy Lesbian.
So, why is this bad? Can lesbians not be in comedies or used for jokes? Not at all. This is a problem because it reinforces the idea that a girl loving girls isn't "real" love. That its a childish phase they will grow out of, or for older ones, a threatening figure that will assault and defile "proper" straight girls. Much the same way that a stereotypical flamboyant gay male depiction is frowned upon due to negative stereotypes, the Comedy Lesbian is effectively that trope, but for girls.
Now, its entirely possible to do comedy with lesbian characters. The key is the punchline should never be "Ha ha she likes girls isn't that ridiculous!" which is, ultimately, what Soleil's character boils down to. And in fact several posters here have said that. Asking why we're getting upset over a clearly comedic character and we shouldn't be taking it so seriously.
A girl liking girls should not be funny. Is the idea of boy liking girls funny? Is that ever a punchline? of course not, because its seen as normal and expected. There are plenty of jokes you can make out of a boy liking girls, through his actions or his words or situations he finds himself in.
The same thing can be done with lesbians. There is an anime, "Yuruyuri", its title can be translated (with some versimilitude) as "Lazy Lesbians". Its all about a group of girls who like girls. But thats not the joke. The fact that they are lesbians isn't ever explicitly commented on or used as a punchline. The jokes all come from how they deal with their budding feelings and how ridiculous love can make people act.
By making Soleil's attraction to women a joke, and giving her only male partners to marry, they effectively are saying homosexual attraction is improper and wrong, and needs to be fixed. This is not something that the writers explicitly set out to do, but its a result of a mindset that doesn't really take the idea of homosexuality seriously as anything but a weird personality quirk.
I hope this helps explain the fundamental flaw in how Soleil was written from the beginning, and how the support with the Male MC is a symptom of the problem, not the problem in and of itself.