Fear of cuckoldry is the diamond market's best friend! Because she'll cheat on you if not distracted by constant sparkling. Or perhaps the idea is that her chasing of other tail will be slowed if she's wearing a sufficiently heavy rock?
Masculinity and Its Discontents is annoyed by jewelry ads too, particularly one in which a good-looking, caring guy is shown painting his girlfriend's toes when the announcer cuts-in with the line "Because you’re not that guy, go buy jewelry at Bob’s." Jamie is quite eloquent as he explains exactly why men should be insulted by the ad:
Jewelry ads, then, in brief, work based on the assumption that men are lazy, have no aesthetic sense, don’t know their partners, don’t really care about knowing them or what they’d like as a gift, and want to spend money as they have to on whatever a clerk tells them their women will want.I'm glad that men are insulted by the suggestion that they are emotional morons who don't know their partners, don't care to, and want a generic 'what girls like' formula to follow for all their interactions with women. Men not being proud to be considered stunted in the compassion department, but actually being affronted by the stereotype... why, it warms my feminist heart (via Pandagon).
In fact, my vote for Most Sexist Ad is insulting for similar reasons. It is a Home Depot commercial in which a guy grabs a female Home Depot employee and says, "Be my wife!" She then follows him around the store, pretending to be his Sweetie on Christmas morning as he presents her with different potential gifts, monitoring her reaction. Because women aren't, you know, individuals or anything. And a husband could never know his wife as a unique person with her own individual interests. Nope - women are a generic brand, infinitely substitutable. (The female employee finally gives the happy reaction when presented with a gift card, by the by - 'cause ladies love shopping!)
And then there's this Lifesaver's ad, which Shameless finds "...patronizing to women, fat-phobic and all around offensive... how messed up is it that her 'supervisor' is making derogatory remarks about her body...":
Copyranter is a whole site dedicated to least favorite ads, and is a good place to start if your still shopping around for your Most Sexist Ad pick.