Although she hid it well, Arnold was also not particularly happy with being cooped up in the apartment. As an angel, it was her job to smite evil, and that was going to require an active engagement with the world, not just watching it on TV. And her job also was to guard against sin, and something told her that her roommates were going to be a handful. Her angelic senses had informed her of the burgeoning sexuality of James and Demelza. (Centaur sexuality seemed to work with an oestrus cycle, and Christian's sexual desire was currently at zero.) Arnold knew that sex itself was a gift of God (although one she had no interest in receiving herself) but it should only be exercised in the context of marriage. She suspected her roommates did not share that belief.
She couldn't coerce James and Demelza--that would have been a violation of free will, and Arnold was an angel from a basically Christian tradition--but she could come along with them and warn them from sin.
"I'll come too" said the angel.
James rolled her eyes. She had a feeling that Arnold was going to be kind of a bringdown.