Via Google, "neighbor's wife" gets 175,000 hits. "Neighbor's husband" gets 7,630 hits. Actual numbers don't matter; it's a question of magnitude: if "neighbor's husband" were a more natural sentence construction, you'd expect it not to be as uncommon by comparison.
Ask yourself: Why "neighbor's wife" rather than "neighbor"?
Lob me a softball, why don't you?
It is a natural sentence construction. I can think of a few reasons within my daily life when I'd use it. Mostly, they boil down to familiarity. I hang out with a lot of married guys, but I seldom encounter their wives. This is for a variety of reasons- some have disparate schedules, some (due to the nature of their work) live separately from their spouses. In general, I'm more familiar with their husbands...some of those women I don't even know by name. One guy's wife I haven't even met, and I've known the guy for 10 years.
And if you ask anyone who hangs out with a lot of "marrieds" you'd probably find a similar phenomenon. "Bob's wife" may just reflect how familiar one guy is with another's spouse, simply because they don't travel in similar social orbits.
The reasons why you get more hits for "wife" rather than "husband" in an online search on that phrase include:
1) That phrase is found in the English translation of one of the 10 commandments. That is going to net you lots of hits, both in the form of discussion of the passage itself, and writers making allusions to it.
2) "Thy Neighbor's Wife" is famous book written by Gay Talese that was also made into a movie- the title is an allusion to the Biblical passage noted above. The nearest equivalent for "husband" is a single episode of the Dick Van Dyke show. Which do you think will get more discussion and thus more hits- a single episode from a famous TV show, or a famous novel AND the movie made from it.
3) There are several porn sites that play off of the aforementioned Biblical passage- I presume in order to be more "naughty"- and we all know how much of an impact that industry has on the proliferation of sites on the Web.
If I bothered to spend real time on this, I could probably get the numbers down to about equal on those searches.