Athos' desperation is clear, but to Faith it just feels like being needed, which is just as nice. The aggression and insistence help her not to think about whether this is morally acceptable, and it keeps her from thinking about the document she signs moments later before sliding the paper to Athos to sign.

She hands him the pen and doesn't process the words marriage license as her hands move back to his body. Breaking contact for even a moment runs the risk of her changing her mind. Athos is a grown man. He can make his own decisions.

And there being no strings attached doesn't hurt, either.