I think both, purely from a financial basis (I work in Finance, shut up) - I believe that both parties are benefiting and therefore they should both contribute.
Stupid story alert
One of my housemates is a girl, who has recently started having sex with her boyfriend. For some reason another housemate (also male) and I brought up the conversation of whether she has a secret stash of condoms in case he doesn't have any. She replied 'No, why should it be my responsibility'. We brought up the case (which is quite often) of when she gets absolutely wasted and ends up back in her room with him, what happens if they want sex but he doesn't have any condoms. 'Well we just won't have sex'. I called bullshit, and it turned out that this situation had actually happened a few weeks ago, they just had sex anyway. She couldn't see why she should buy the condoms, despite them both participating. 'Well, sex isn't that important to me really, I can go without'.
This conversation went around in circles for a couple more times...
idiot