I counter the first response. As opposed to other film stars of his day, much of Cary Grant's appeal came from his ability to not be a manly ideal. In many of his early films like Bringing Up Baby (1938), My Favorite Wife (1940), Arsenic and Old Lace (1944), and Mr. Blandings Builds His Dream House (1948), he plays a nerdy or hen-pecked person who is constantly outmatched by the situations and characters around him.
Bringing up Baby
Cary Grant reduced to wearing Katherine Hepburn's bathrobe in Bringing up Baby
This is not sexy
The Howards of Virginia
In these films, he's shown in bathrooms, or unclothed, or shaving because it emphasizes his vulnerability and makes him funnier. It's actually particularly true in North by Northwest (1959), where there's an entire sequence about the indignity of him having to use travel-sized shaving tools (while on the run).
North By Northwest
In the specific case of Mr. Blandings Builds His Dream House, the bathroom is a metaphor for Grant's characters needs throughout the movie.
His attempts to fight for turf in the bathroom mimic his efforts to get what he wants out of the giant remodeling at the center of the picture.
Bathroom scenes are moments when Grant, as his character's life is spiraling into ruin, attempts to put his life back together and fails.
Arsenic and Old Lace
Okay, this shot may be for the sexy.
Also this one.
If anyone would like to try their hand at a general analysis of actors in tubs, here's a great image resource.