Edit: I rewrote this due to the comments
Let's say that at the beginning we are at an equilibrium state in which the couples are perfectly symmetrical, and there are no different behaviors that can be regarded as gender roles. Now, outside forces on the couples could push them toward a gender specialization such that one partner is like you said the breadwinner and the other the homemaker.
One major such force could be society. If it's the norm to have gender roles, it's highly likely that even small kids would adopt a gender identity or at least consider what they prefer and have it influence their process of finding a mate and their behavior in their relationships. But something has to be the first trigger.
Option 1. Biological constraints (i.e. pregnancy)
In the limit case in which bearing a child is extremely easy and takes only few days, it makes little difference which of the couple will bear the children, obviously the couple could alternate who bears the children and no difference in behavior is to be expected. This isn't the case, since pregnancy is 9 months long and taking care of small children is also a handful, so this is definitely a force that would push the couples to specialize, one in childcare (and from that to homemaking) and one in their career.
Option 2: Economical constraints
What if in this society one paycheck is more then enough, and people don't see a need to earn as much as possible? (This can be due e.g. to strong social networks, religion or lack of ways to spend a lot of money) What if at the same time a very well-maintained house was considered a big virtue? This could easily lead to a joint decision to split the work.
This can be combined with the first option to make it more difficult for someone to have a child (thus be out of the work force for a while) and then resume their career.
Option 3: Personal preference
Let's not forget that even in same-sex couples there may sometimes be something similar to gender roles, and in heterosexual couples the roles my switch. So a personal preference or natural talents could also play a role.
Since the existence or lack thereof of gender roles is something that perpetuates itself, I think it's a matter of equilibrium. A trend in either way will amplify and become the norm, until a trend in the opposite direction changes things. The fact that genders in your society are not based in biological differences makes change all the more easy and therefor plausible.