Well, I would never pick the sex of my child. I won't tell someone else that they can't, though. However, I do think it's a bit like playing God.
As for finding out the sex ahead of time, it was simply practical for us to know the sex so that we could plan ahead. I have three baby showers coming up and did not want my child to end up with a million green and yellow items simply b/c no one knew what to give us. Plus, I am really not patient enough to wait it out. I don't know how my mom made it (except that the same way I knew I was carrying a boy, she knew she was carrying a girl).
ETA: I just wanted to note that we found out the sex at the ultrasound... We did not PICK the sex
LOL, husband thought it read like we had picked the gender...