Well, even if my birth control argument is disproven (even though it wasn't part of my argument, just a character aside) I still maintain that its probably not common practice to go through several lovers in a clan. There has been a decent number of exclusive partners mentioned, so I'm assuming that's the norm.
Of course this is all just speculation on my part.
I'd guess that depends more on the personality and willingness of the people involved? The only barrier I can see to multiple lovers is the fact that you've got a rather limited dating pool when your clan isn't that large to begin with. It'd be like being at a very small high school. I can see a lot of serial dating happening between people in the same age range, though. And if something too embarrassing happens then you always have the option of electing to go to another clan for a while.
This also makes me think that the Arlathvhen is a time when a lot of new couples form. It's the first time a lot of these young people have access to a social circle bigger than the people they grew up with.
I can actually see there being a lot of experimentation quite early on within Dalish clans. The structure reminds me of desert Bushman tribes, where sexual activity between age-mates begins quite early (before you have to worry about pregnancy). Once a girl is able to get pregnant, however, she's married off. But since the Dalish have reliable birth control, marriage can be put off.