If the swap happened when both Marikas were kids, I guess the boyfriend had to be a childhood friend or something or else it would make no sense that he would feel something off about yuri-Marika compared to straight-Marika because he never would've known straight-Marika to compare their different behavior in the first place. Guess we'll never know because they didn't actually bother to describe or flesh out the boyfriend at all despite trying to use him for some cheap drama at the very end only to resolve it in the next page instead of leaving it for an epilogue chapter or something.