Here's a very important thing...
Mary wants Sherlock and John to still be best buddies. (Way different from how John’s previous girlfriends have reacted to his very committed friendship with Sherlock) She wants them to feel free to show affection, even encourages more affection. She WANTS them to go solve murders. (And she’ll even join sometimes because solving murders is fun, but not everytime because she knows they need their own adventures and time to just be boys)
And Sherlock makes an effort to be nice to her. And more importantly, he actually LIKES her! She’s smart and useful and loves John just as much as he does.
And John acknowleged that they have bothed saved him and just loves the shit out of both of them because he’s a warm, cuddly, loyal, open hearted little soldier boy.
THIS is what it looks like when there’s a pair of best friends, and one gets in a relationship, and it doesn’t automatically ruin everything! They believe in their friendship and she believes in their friendship and creates her own friendship and everything is good!!
BEST FRIENDS CAN HAVE LEGITIMATE RELATIONSHIPS THAT DON’T RUIN THE EXISTING FRIENDSHIP.
And not ofen enough represented in the media!
(This is disregarding the end. Which in my opinion, was more caused by the new baby, which changes all the relationships in a whole new way.
And as far as Johnlock goes, there’s still no tension. If you do see a romantic relationship there, I’m convinced they just ADDED Mary to their love nest. Still not endangering the original relationship. Bam.)