Justified: The series or movie is set during a time (such as The Fifties) or place where condoms are/were sold as a behind the counter item (rather than an over-the-counter item.)
Bob is new at this; it's natural to be embarrassed the first time. Next time (if there is one) will be easier.
Inverted: Bob is proud to buy condoms. The pharmacist is embarrassed.
Subverted: Bob goes into the shop looking furtively over his shoulder, sidles up to the till, and... "do you have any.. chewing gum?"
Double Subverted: "...oh, er, And Another Thing - some johnnies, please"
Deconstructed: If Bob feels too self-conscious about buying condoms, he might decide it's not worth it, and have unprotected sex (or try improvising a condom out of plastic-wrap or something), resulting in STDs and unwanted pregnancy.
Reconstructed: Bob gets over the angst, and marches up to the cash register with the condoms in hand...and finds out that he didn't have anything to worry about, as the cashier simply rings up his order.
Parodied: As Bob (already embarrassed) buys the condoms, something bad happens, and everyone blames his buying condoms for it.
Lampshaded: "Oh, crap! I'm here buying condoms, and there's my math teacher in the same checkout line!"
Enforced: The writers want to give An Aesop that buying condoms isn't nearly as embarassing as their teenage viewers might imagine.
Invoked: Bob uses this as an excuse not to buy condoms - really, he just doesn't want to use one.
Exploited: The pharmacist sees himself as a Moral Guardian, and tries to use teenagers' embarrassment to scare them out of buying condoms at all (that'll stop them having sex for sure!). They're all sold as behind-the-counter items, and he asks things like "Do your parents know you're buying these?" or "Are you married?"
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