On my straw page, it shows that I have 46 outstanding straws for one stud. What exactly are outstanding straws? My best guess is that "outstanding straws" are straws that other people own (but you own the horse). It says I can only have 50 outstanding straws at a time so if my guess is right, does that mean I can only sell 4 straws to other members?

Yes, you can only draw 4 straws at a time and have to wait until some straws get used before drawing them again, if someone owns other 46.

You can always ask if a player or players would be willing to sell some of those straws back to you or use them instead.