[ Part of this is knowing that technically, someone could walk by and catch Ianto making weird faces at himself. Much like fucking while someone's on the phone (another thing they should absolutely do) there's a thrill to knowing Ianto needs to maintain a certain level of self control, lest he gives himself away.
So, naturally, Spike decides to test this by biting into the soft skin of his inner thigh. Not hard enough to break skin, barely hard enough to even leave a mark, but that's not the point today. ]