Mary Steenburgen says she thought her husband Ted Danson was a cool guy before they met ... until he punctuated a roll in the hay with a lame phrase.

The actress revealed the news to People at the Golden Gala red carpet on Friday ... explaining she always thought TD was a pretty cool guy because of his onscreen persona.

That all changed after they first had sex, MS says ... 'cause when they finished, she swears Danson couldn't help but say "gosh-a-rooni."

Mary jokingly apologizes to her granddaughters for dishing on the post-sex phrase ... 'cause they probably didn't want to get to know their grandparents that well!

Steenburgen says there was a lot more to Danson than she thought when they originally met ... and, over the years she's learned more about his activism and kindness.

She says his interaction with her kids defintely helped build up the love between them ... and, she called going through life with him "thrilling."

Ted and Mary first met back in 1983 ... though they didn't marry until the mid 1990s. They have children from previously relationships but none together.

Moral of the story ... never judge a book by its cover -- or a person by what they say just after sex!