Do people do this with other female heroes? If so, folks might want to think about why they think women characters only can only be strong if they're unattached romantically.
I don't know if this was asked genuinely or rhetorically, but here is my view on it. Rather than being any one thing, it's actually a couple.
1. Traditionally speaking, the arena of love has been generally where women have been holders of power. This is rooted in that a woman's primary social value was her marriage potential, so if a woman was loved by a man, she had power over that man. This was the premise of many a victorian romance novels, such as Wuthering Heights, where women only had so much power as the men who loved them.
2. The other traditional aspect is that in Superhero comics, the love interest is typically part of the fantasy of being the empowered male. I mean, what is the point of having super strength if it also doesn't get you the attention of your crush? Which it should because you're using it to save her.
While 1 can be empowering for women in a way, at first glance, it might be interpretted as a relapse into the old social structure. Hence the rejection of a love interest because a truly strong women would transcend the boundaries set by society. She wouldn't need to romantic power because she'd have physical and/or political power.
2 is much less sophisticated and not willing to look how engaging with a romantic partner can be just as fulfilling from a woman's end as a man's. I think it's partially due to the fact that a lot of stereotypical superhero romances are premised on the women being unempowered. Superpowered men come and save her from the plane crash, from other men, etc, and then she loves you. If wonder woman were to do this to steve rogers, it'd be considered emasculating (I hate this word, because it equates self competence with masculinity, which inherently implies competence is a male trait, but it's vaguely appropriate here I guess) and be met with revolt. We get so few fully fleshed out romances between men and women where they have equal agency that when asked to imagine one from a woman's perspective that doesn't disrespect the male partner, it's harder to do.
For the most part, it just means we need more well written stories with females to understand, yes, what a shock, you can have both a power fantasy and respect to the romantic relationship. Not having that would be an easy way around wrestling with what is a percieved insurmountable issue, when in actuality, a superhero romance for a woman can be just as satisfying as a superhero romance with a man...
That said, Superhero romances are rarely satisfying for me in general. The only ones I can think are subversions in some way. Rachel and Bruce from TDK because Rachel ultimately went with someone else. Granted, she got fridged, but I liked that she wasn't beholden to bruce as he went around his crusade. I also like Carter and Captain America, but again, their romance is a tragedy and short lived, and their stories are more interesting about how they deal with the loss of their relationship rather than the relationship itself. I don't know what else is good out there, unless we start bringing in comics into this.