Not being Islamic, I don't know anything about the winged-horse thing, but the virgin birth of Jesus is entirely possible without being at the same time a huge physically present miracle, which is in my opinion, exactly how God works, in mysterious and un-obvious ways. The bottom line is that the virgin birth is a matter of faith, but that self-reproduction in organisms is not unheard of and though not a typical manner of gestation with respect to human beings, entirely possible. If one believes in God, and that God works through the normal scientific processes, it becomes then more probable to a believer.
It's all faith and a matter of opinion, but if you're a non believer I wouldn't simply dismiss it as dumb. I'm not saying you are--you're just making a point--but many people do.