I know tv tropes like to point out that the Phineas & Ferb character of Baljeet may be gay, judging by his interaction with Buford. And according to Craig Bartlett, Mr. Simmons from Hey Arnold was gay. So we have already witnessed gay characters in western animation, just not a character coming completely out of the closet and saying it on screen. Then again it's hard to find a gay character in live action or animation who is not portrayed as the stereotypical flaming homosexual either.
(Not animation, but this time around I think Degrassi is doing a great job of writing for their two gay characters, Riley and Zane.)