Valve have a Steam blog up, which explains the process for age verification and the need for a credit card in particular.

Valve claims Mastercard cited a rule to force them to change NSFW game rules on Steam, disputing Mastercard's attempt to deflect blame.

A mass purge of sexually explicit games exposes the little-known yet all-encompassing system of ‘financial censorship’ practiced by payment providers such as Visa and Mastercard, reports Io Dodds ...