I wouldn't want Bert and Ernie to get married, but it has nothing to do with homosexuality. It has to do with change. This is a show for children, and changing characters is generally a bad idea. When Sesame Street dealt with Mr. Hooper's death, they had to, because Hooper was a staple on the show and he actually died, so his disappearance was going to be a disconcerting change. So they dealt with it, which was responsible. They didn't decide to kill off the Hooper character in order to teach kids about death.
I think it would be great if they introduced a new pair of married men (or women). Introducing new characters is much better than changing established characters. Sesame Street introduces new characters all the time. Elmo was introduced in the 1980's, Zoe in the 1990's, and Abby Cadabby in the 2000's. Introducing a pair of gay human characters, especially if they're introduce simply as the new neighbors and accepted as if it were nothing unusual, would be great.