Well, she already likes Jack, and really, what's not to love about a hottie with brains AND good sense? And the old girl is probably a voyeur at heart who will fsck with all the hormones of everybody else inside once those two boys get to shagging.

Can't you just see it now? Donna and Doctor trying to figure out why they're shaking and weak at the knees, and meanwhile Jack and Ianto are having TARDIS-enhanced telepathic sex? And Martha (who comes along for a ride) laughing at everybody else?