Just thought I'd mention that there are species of fish and other animal groups that are all female or close enough to it that their populations do not rely on males to propagate. For example, Amazon mollies. I couldn't tell you if any species of betta are capable of parthenogensis, but as a general statement "If you would have only females, the entire fish species dies out" isn't universally true.
Although it's possible that female bettas want strong males, I'm certainly not a female betta and don't want to get too far into the territory of anthropomorhpism and there are certainly other possible motivations and processes going on than simply wanting "strong males".
But really, all this seems to be veering oddly away from the original topic and my first comment, so I'm just going to leave it at that.