Darius, is this a real question?
Why do any guys let their female companions choose their clothes, their underwear namely? They wouldn't let their female counterparts choose their shoes, I think, but that's about it. Guys don't like shopping, for one thing. Second reason, guys are too immature or too lazy to go and buy their own condoms. But the females of the species are the ones who've got to deal with pregnancies, if they go without a condom. So once a trend has set in, for example the wife buys the condoms, you let her get on with it. I wonder how she's to know that the condom doesn't fit, though, if you ever tell her. I think the pharmacists may not be doing their job informing the customers?
As a reminder, there's the famous scene in Summer of '42, which explains how awkward buying condoms could be.
"It’s just one of the many important questions this film asks its target male adolescent audience. How do you control arousal in the presence of an attractive woman? How do you know when it’s the appropriate time to put your arm around her at the movies? How do you know, without looking, if you’re squeezing a breast… or an arm? How do you hide your embarrassment when buying your first condom? The latter makes for what is perhaps the most hilarious scene in the film. Oscy and Hermie are preparing for a campfire date with a couple of girls their age, and Oscy has dared Hermie to buy contraceptives at the local drugstore. Mulligan plays this scene until it’s absolutely side-splitting: first Hermie has to wait for an elderly female customer to leave. Then he has to ask the druggist (Lou Frizzell, one of Mulligan’s favorite actors) for a triple strawberry ice cream cone, then some sprinkles, then a napkin, and then “HOW ABOUT SOME RUBBERS?” Which brand? the druggist asks. How many? Who’s he buying them for? Hermie by this point is so overwhelmed with options, the druggist finally has to cut to the chase: “Alright now, son, fun is fun, but how old are you?” He asks Hermie if he knows what condoms are used for. “Sure,” Hermie responds. “You fill them up with water and then you throw them off the roof.”"
It's at the 58th minute if you can find the film online.