From Wikipedia:

Frank Oz, who previously performed as Bert, stated Bert and Ernie were not gay, saying,

"They're not, of course, a gay couple. But why that question? Does it really matter? Why the need to define people as only gay? There's much more to a human being than just straightness or gayness."[17][18]

Sesame Street went on to clarify further, stating:

"No, Bert and Ernie aren't gay - they're 'best friends'".[19]

The Gaystar News reported that fans reacted negatively to this assertion. Frank Oz later tweeted in September 2018

A last thought: If Jim and I had created Bert and Ernie as gay characters they would be inauthentic, coming from two straight men. However, I have now learned that many view them as representative of a loving gay relationship. And that's pretty wonderful. Thanks for helping me understand.[20][21]

So, I would say they weren’t intended that way from the start but only because the writers didn’t think to do so. Frank Oz is a great reference for these things, and if he says “I love how you see them as representation” then that’s good enough for me.