Personally I don't think forcing the issue is the way to go especially if you are unsure. I would only go ahead if you are completely relaxed about all the issues you have mentioned. If you are feeling uncomfortable your prospective partner will pick up on it and that can lead to complications especially if you decide at the end of the day you don't want to go ahead.

It should be a wonderful experience not one fraught with what ifs and angst. And certainly not one where you rely on a tick in the box from us. Ostensibly it's between you your prospective partner and both of your "married partners." If it's ok with them and you that's what you should be considering because if things go sideways as they can that's who will be directly affected.

Someone wise said "you don't know what you've got till it's gone". Making up our own minds is one of the few true freedoms we have left. Exercise that right Hime, follow your heart it knows best.

Good luck and enjoy whatever you choose.