Oh my goodness, my BFF sometimes gets annoyed with me when I write these interjection words the wrong way in e-mails! I write "yea" (a sound of excitement) as "yeah" with an exclamation point on the end. But I also write the "yeah" (of agreement) as "yeah" with a period on the end. So that can get confusing for people. I also write "whoa" as "woah." I even had to look at your spelling to check what the "correct" way to spell it is, since I've always spelled it "woah."
As for how sounds should be spelled during sex, I think I prefer reading "he moaned" vs. something like "Uhhghnn." I don't know, I think that most times it's better to imagine what the moan sounded like rather than trying to describe it.
Anyway, this post was really interesting! I really enjoyed it! =)