Barnabas commented sarcastically on her attraction to him back in the vampire days when he was still semi-throttling her. It was really our first indication of it. That certainly came out of nowhere....
perhaps <---
, julia enjoyed being throttled by Barnabas, because i think that he started her on her masochistic relationship with him. it branched out to other people throttling her and choking her.....thus her gravely voice.
where she originally starte dout as work partners with him, she grew to love him, beign sparked by his affections for sniki viki. from there it was an uphill battle for his attentions.
i still say, they should have remained a non-couple. sex and relationship problem would have complicated their holmes and watson teamwork and friendship.