Teen,
Look, if you believe god created us then it would also make sense he created us for something.
It makes sense he would tell some people what that reason is.
So the idea of the story of matan torah itself (that god came down and said what he created us for) would be kind of bound to happen (of course this is all going with the assumption that in your head it is already proven that god created us) .
So if there is a story that god came down in front of 600,000 men and their wives and children, and told them what he created us for,
since god is bound to come down and tell us why he created us in the first place, and this is the clearest story we have of god coming down in front of an entire nation that we have (which would be a very hard story to make up) there is really no reason not to believe it.
Now last I checked if a woman becomes expecting there is only one way that happens (and it is not because god just planted a baby in her stomach).
It also makes sense that she would deny having done anything that would have got her expecting right? so unless you can prove to me beyond a shadow of a doubt that yoshke was not just a mamzer (which is not possible) there is no reason to believe he was anything more then that.
(P.s. I know that this is not really such a great answer but its worth a shot and may be interesting to discuss i think)