I don't really love the 'dating' episodes where often Mary stops being funny and instead goes all soppy. The guys are often real catches and the episode ends on a hopeful note, but usually we never hear from the guy again in any later episode.
This one is a dating episode in essence, but it is dressed-up with comedy elements that do work quite well.
Bradford Dillman is charming handsome (and forward) architect Matt Bryan who is studying Mary's apartment house. They quickly hit it off.
There are a few story ideas comically explored here: Everyone marvels at Matt's youthful appearance (Bradford Dillman really is about the same age as Ed Asner and Gavin MacLeod). Then there are surprises when his son (John Rubinstein) is much older than expected. A third thread is less comedic and explores the idea that Matt Senior keeps the relationship with Mary superficial due to fear of committment, then nervously proposes marriage.
Dillman and Rubinstein play their parts well.