It seems astounding, but no more such than creatures having two eyes “mostly”, or four appendages “mostly”. Although asexual reproduction is possible, and frequently happens, it doesn’t allow for the same diversity as sexual reproduction. Technically, a creature or plant that can reproduce asexually can impregnate or pollinate itself. Think of hermies in a cannabis plant. Much more stable, and robust genetics come from two living things sharing their genetics.
With that in mind each creature needs to excel in passing on it genetics. Think, crudely, of a giver and a taker. One excels at giving its genetics, while the other excels at taking foreign genetics. It makes sense that the better a creature is at one or the other the easier it is to become impregnated. However, the ability to do both increases the chance of self impregnating, which would cause more genetic anomalies to occur.
I’m not a scientist, but this is what makes sense to me.