Well Jamie knows the marriage is a marriage of convenience, but he doesn't know that Claire doesn't plan to stick around once she can get to the stones so as far as he's concerned the marriage is real, despite the reasons for it. So Claire either tells him she's planning to escape, or she goes along with acting like it's the real deal, which includes consummating the marriage. And since "sex with Jamie" is behind Door Number 2, of course she picks that one. ;)