First off, it may well be an unauthorized shirt. I can certainly believe that. If it's not, or even it is, I still believe that shirt is as gay as you want it to be. Sometimes a rose is just a rose.
It can be just two best friends and a rainbow, symbolizing the sunny days that are enjoyed on Sesame Street.
I guess it's nice to see a letter from Sesame Workshop addressing the Bert and Ernie rumor and all, but honestly there's no need to keep berating this issue. It's been officially stated for the record, so no one will keep wondering, that Bert and Ernie are not gay.
Why is the concept of best friends and roommates so hard to understand? They are most certainly a great pastiche of Felix and Oscar from The Odd Couple. That's a perfect example.
Plenty of guys in colleges all across the world room together in dorms and plenty of guys continue to room together in apartments after college before they get married. There's nothing weird about it and no need to speculate.
I like how Sesame Street explained their educational purposes for Ernie and Bert- they're a great example of how people can get along despite their differences. This is a good picture of what tolerance should look like really- being able to accept that people believe different things and in fact, are different in their own ways- and though you may not necessarily agree with their beliefs or choices, you can still accept them as a person and listen to them out of respect and love for all. (Because we should love and respect everyone- we're all made in God's image and that's reason enough. And we should certainly do as Jesus told us in Luke 6:31 and "Do unto others as you would have them do unto you.") It's being able to accept that Ernie likes to be loud and noisy and kinda wild and crazy and Bert likes to be quiet and peaceful and a bit straight-laced- but both of them can accept that about each other and still like each other's company because they're best friends.
I think their "But I Like You" and "That's What Friends Are For" songs sum it up best.
"I don't like any of those things- but I like you."
"I don't mind if you like .....- that's what friends are for."