While a commitment ceremony is based on a traditional wedding, so many traditional wedding customs are inherently based on the mixed gender of the couple. There are a number of ways in which this can be addressed for same sex couples.

I do not believe it is appropriate for the partners each to take on a heterosexual role. Instead, I suggest that you assign individual customs to the partners instead of entire roles (customs includes who enters first/last, who stands on the right, who takes the first vow, and so on), and also assign customs in duplicate (for example, both partners walk down the aisle instead of just one).

In planning a commitment ceremony, you can start with a blank sheet. There are no hard-and-fast customs that everyone believes you MUST adhere to (well, apart from the ring and the always-important kiss after the declaration). As a result, you can do whatever is authentic to you as a couple. You can have whoever you want stand up with you, and you can enter the ceremony space accompanied by whomever you wish.

In terms of your attendants/supporters, there is already a precedent for mixed genders in straight wedding parties. Because more and more people are having the people they are close to stand up with them, rather than sticking with the traditional gender divide, we are starting to see a neutralisation of the gender of the roles by referring to the matron/maid of honour/chief bridesmaid and best man as "honour attendants" and bridesmaids and groomsmen as "attendants".

The gender bias in straight ceremonies is most obvious in the way that a traditional wedding party