Chivalry or Sexism?

The other day, I was on the bus home from college, and was chatting to a few friends about this and that, and the topic of sex came up. I jokingly made comments about purchasing condoms from Amazon (which may lead to another point later)**. A girl who I don't know, who was sat behind my friends and I, piped up and said "Well, why do you have to buy condoms? Why doesn't he?" (By 'he', I assume she meant my boyfriend.) I simply replied that I don't have to, I choose to.

I don't understand why it is in our expectations that the man has to buy the condoms, and why in supermarkets and pharmacies, condoms are often found in the "Men's Health" aisle. Yes, I understand that men are the one's who wear them, but most of the time it is for having sex with a woman - although obviously gay couples practice safe sex too. But the point is, is that sex is (should be) a mutual thing, so why shouldn't the woman purchase condoms?

Don't get me wrong, I don't always buy them, my boyfriend does too. But it works exactly the same way as, sometimes I'll pay for a food bill, sometimes he will, and other times we'll split it. I don't have a problem with buying them, and neither does he. It just confuses me as to why people expect men to have to buy them, women have sex, so why is it 'weird' for us to purchase contraception? This girl also asked why I don't "just go on the pill", well: Condoms are more effective, protect you from STIs (not a problem with me, but for others, a reasonable factor), they don't affect your body in any way (as the Pill does, as it is a hormone pill) and also, for some people, The Pill doesn't work. But her reason was that it "Saved embarrassment from you buying them." (condoms). I just wondered why it's perfectly plausible for my boyfriend to buy them, but me buying them would obviously cause embarrassment?

** Another quick one, why are there barely any differences in the sizing of condoms available in pharmacies and supermarkets etc.,? It would be like only having shoes available in the two or three most common sizes, unless you went on the internet. We need a bigger range of condom sizes more widely available, to cater for everyone.