Relena's train of thought: "Are these shallow fangirls THAT mad because I might get some?"
The Gundam meta-series is a BIG long runner with many different continuities. The bashing started with Zeta Gundam, became mainstream with G Gundam and Gundam Wing, and ever since then it just won't stop.
Technically, this happens to most of the show's female cast, since there's a 1-to-1 ratio of men to women among the primary cast — even ones who don't express any overt signs of romantic love for their counterparts and seem to go more for friendship or partnership with the guys. Relena gets it the worst (by far) because of a bad first impression. Other common victims are Duo's partner and potential girlfriend Hilde; Treize's right-hand Lady Une; and Quatre's rival/counterpart Dorothy, who also receives hate for opposing and hurting The Woobie during a fight late in the show. Trowa's Cool Big Sis Catherine Bloom gets surprisingly little of this, perhaps because she and Trowa act more like siblings than potential lovers. It has been established in recent years that they are, in fact, blood siblings, and see each other as such.
Ikeda was only one of five script writers for Wing; three of the others penned side-stories which blatantly ship Heero/Relena, including Blind Target (where it's implied they actually kiss), Battlefield of Pacifists (where both Duo and Dorothy attempt to set the two of them up together) and Frozen Teardrop (where Heero and Relena are cryogenically frozen for 30 years, rendering any potential relationships with the rest of the cast Squicky).
But the last one has a big detail... Relena is now a Brainwashed and Crazy sort-of Barrier Maiden, and it looks like Heero will have to kill her in the end. As the corollary of several Star-Crossed Lovers deals involving people who, for one or another reason, look a LOT like Heero and Relena themselves (the original Heero and Katrina Peacecraft, Ein Yuy and Angelina Khushrenada, etc.) So even with the aforementioned deal, poor Heero and Relena might not end up together either. (Or it'll take them LOTS of blood, sweat, and tears.) So yeah...
And it's not only about the infamous AsuCaga shipwar. If you like Kira/Lacus, apparently you can't like Fllay and must call her "whore" and "rapist". If you prefer Kira/Fllay, you're often expected to call Lacus a plain and unrealistic Purity Sue. Not all the fans are like that, but it's still often the first thing that other fans see when the Ship-to-Ship Combat is mentioned.
A case that's more or less forgotten but was horrible in its time, is the war between Domon/Allenby and Domon/Rain shippers in the G Gundam fandom. Allenby/Domon stans bashed Rain for being "girlier, therefore weaker and a slut" and "too whiny"; Domon/Rain fans retorted by calling Allenby a "third-wheel bitch" who "wanted to tear apart the Official Couple". Nevermind that Ally pulled an I Want My Beloved to Be Happy and pushed Domon towards Rain in the end.
What makes the Rain vs Allenby debate even more interesting is that Koichi Tokita, the author and artist of the official manga adaptation, went on record as saying he preferred Allenby. However, rather than making Rain look bad, he tended to make her look good but Allenby looked better, both in the manga itself and in the silly Yonkoma included with it.
Sadly, it went in Feldt's "defense". More than a few Feldt/ Lyle Dylandy (Neil's Backup Twin) fans bash Anew Returner HARD and call her a Canon Sue for threatening their OTP. Which is ironic and utterly stupid, since before Anew ever was in the picture, Lyle actually preferred to put on a Jerkass Fašade in front of Feldt and push her away, rather than encourage her to pursue his affections. The fact that Anew in canon hooked up with Lyle, turned on the crew (even if as a Manchurian Agent under Mind Control), and wasn't convinced that her time/feelings for Lyle were true until it was way too late doesn't help fangirls' negative perception of her at all, even if that's not all there is to her character and that whole deal wasn't exactly her fault, but her "twin brother" Revive and Ribbons's own And of course, these two men get scot free in fandom view.
Setsuna turned out to be the standard Gundam AsexualCelibate Hero... until The Movie's distant finale. He visits an elderly Marina and, according to The Movie's manga adaptation, Setsuna restores Marina's youth and ultimately marries her, much to the chagrin of the Setsuna/Feldt shippers.(Though there are doubts about how canon that one is...)
Miraculously, MileinaVashti has managed to dodge shipping-based hate as well, despite having a crush on Tieria and confessing to him.
And believe it or not, not even the Universal Century Gundam series are free. The hate that Zeta Gundam fans levelled to Fa Yuiri came less from her being a Na´ve Everygirl stuck in a bad situation and not being able to become a 100% Action Girl, and much more from her "being in between" Kamille and Four's True Love. Even more so after Four actually dies in really tragic circumstances, Kamille lives on but is mind raped into catatonia, and Fa lives on too and stays with him as he slooooowly gets better.