Not by itself. It's more the fact that almost whoever she S-supports with gets them both stuck in the friendzone, it's not like they gave her different marriage options.
Also I'm 100% against trivializing a character's sexuality. Exacerbating their quirks like in Effie's or Rhajat's case is annoying enough, but this is not ok.

Also because I shipped her with Siegbert, but you needn't know that.

The thing is…..Soleil is not straight, and wasn't straight She's Bi. Now, the real problem in the original japanese version was how she was treated: She's pretty much a pervert towards girls, and was sold as being Bi (in her own words, she has no problem with men, just prefers women), yet ironically could only S-Support with men, with her obsession with girls being treated more as comedy, making her rather one-dimensional since her whole character is based on that.

Now, the only reason I can imagine for WHY that was the case, is this: Japan, is very backwards when it comes to this. Gays and Bis are seen as nothing but jokes at best, inmature people who need to grow up at worst, the yaoi and yuri genres are rather niche there. That in itself explains Soleil's character, why her love for girls exists only for comedy.

Now, why could she only romance guys despite her clear obsession with girls? Again, because Japan is very backwards in that aspect. There already were 1 lesbian and 1 gay option in the game, which is actually progressive by Japan's standards, but Nintendo either didn't want to risk too much or thought there were more than enough gay options.

I believe this is the reason she was reduced to only Foleo and M!Corrin as romance options. To make her "case" less questionable. She was sold as Bi yet could only romance men, that's an idea that not many westerners like, so they reduce her romances to those 2 to make her Bisexuality more believable.