No, they are not married yet (till the end of Infinity War).
They announce their engagement to public at the end of Spider-Man: Homecoming, when Peter denies to attend the press conference where he was supposed to be declared part of The Avengers.
Pepper: Where's the kid?
Tony: He left.
Pepper: Everybody's waiting.
Tony: He actually made a mature choice.
Happy: It just surprised the heck out of us.
Pepper: Did you screw this up?
Happy: He told the kid to go wait in the car.
Pepper: Are you kidding? I have a room full of people in there waiting... ...for some big announcement. What will I tell them?
Tony: Think of something. How about...? Hap, you still got that ring?
Happy: Do I-- ? I-- ?
Tony: The engagement ring.
The dialogue said by Dr. Strange goes like
Oh. Congratulations on the wedding, by the way.
There is a possibility that Dr. Strange heard their conversation and then he guessed that they might be married since Tony was talking about naming their child.
Another possibility could be that Dr. Strange might have seen Tony's future and saw that Pepper and Tony are married. That's why he says that. Though, this is a bit stretched, but my other answer has an interview of Gwyneth Paltrow which confirms that they will be married in future.
And the second dialogue by Tony Stark means that they're about to get married and Wong is invited to the wedding. Therefore, it implies that they are not married. Engagement and wedding are two different relationship statuses.