I think in Origins it was implied that there was a life time vow and that would explain the other Templar with the desire demon. Then when dragon age 2 came out they kinda retconed it ( I'm not 100% on that).
I also had no Idea that Sebs vows expired! I thought that his ones where for life. Maybe I should reconsider that romance on my next playthrough.
I think Leliana explains that in DAO. I don't remember exactly but I think it was something like this:
lay brothers and sisters (no vows) that's what Leliana and Seb were and Seb was about to take the vow
brothers and sisters with vows (brother Genitivi)
mothers
reverend mothers
Divine
I also think they retconned the Templars being allowed to marry. In DAO it sure seemed like being married was forbidden in that scene with the demon but then you just run into Aveline and Weasley in DA2. I think they should have explained more about Weasley, by act 2 I just forgot all about him and Aveline never talks about their life together or how they married.