Steve walks through Robbie's room with sadness after he and Katie have left for their honeymoon, officially marking Robbie's departure from childhood and on to adulthood for both him and Robbie.

Does Steve do this for Mike when he gets married?

If he does we weren't invited along.

When Steve is on his honeymoon, Rob and Katie inspect the room he and Barbara will take when they return (which was previously Rob and Katie's) and Katie comments on the arrangement, saying what she would do (and Barbara eventually does what she says).

Thanks.
It seemed so sad for Steve with Robbie. Steve didn't seem so sad when Mike got married.

If Tim Considine hadn't left the show so quickly they could have made a longer story arc that began the sixth season. That could have included Mike's wedding--more that the minute or two they devoted to it as it was--and Ernie's adoption.