im pretty sure they don't have the balls to make a bisexual MC.
That said while i personally prefer my harems entirely made up of ladies i would not be against it.
@monkey123 this one is categorized as a shonnen and a pretty mild one. They may already done it behind scenes or not. But, in reality, it changes nothing. MC and mermaid lady are already adults only want each other. So no room for moral, ecchi or tsundere misunderstanding. Neither of them want a harem and, currently, they already call each other spouse