I agree that they need to date other people. I think if Steve had an anchor into Diana's life like Catwoman and Lois we wouldn't have to worry about whether he can be erased by a new relationship or letting Diana explore different options but because she doesn't have any normalcy for fans to cling to you always have the possibility that he will go missing for another 25+ years if not indefinitely. I think if writers focused on establishing stuff like Argus and Holiday College as important bases for Diana's world there wouldn't be a big worry about her dating other people or having new cast members because all of that would be established.
Bruce has dated plenty of other women and I think has a different love interest in every movie besides two where Catwoman is his main love interest. However, Catwoman has and probably always will be thought of as his most important and his "endgame" because of how important she is. Lois is obviously going no where and has held that down for decades, but Superman has dated here and there.
The idea that Diana dates the first guy she meets and then runs off in the sunset with him for the rest of her life is already suffocating to deal with but the fact that the relationship hasnt existed for decades makes it harder to really believe. I haven't enjoyed the relationship between those two much at all even in the film that everyone loves so I'd like there to be more there.
I think its more believable to have her have at least one or two relationships before she leaves the island and then a few others after. I don't hate the idea that she was with Batman for a moment, or superman if you're into that sort of thing, and then went her own way when it just doesn't work out. I'd even like her to have a past with Angleman or some other equally sleazy but not repulsive villain.