Can nonmonogamy help a lowered libido in a LTR?

Ok so this may be a long post, please let me know if anything needs clarification.

My (23F - bisexual) boyfriend (24M - bisxual) and I have been together for 3 years. We both talked around being interested in group sex or opening a relationship early on and both want to explore our bisexuality which we have not yet done. We very much want to stay together but also would like to explore.

The other part in this is that I tend to lose my libido in a relationship about 6 months to a year in. I have vaginismus which makes sex very painful and have had it for about 10 years (possibly as a result of sexual assault.) But I find that when I'm with a new partner, I am too excited to mind the pain so I can be my normal sexual self.

My current theory is that as I stay in a long relationship, the honeymoon phase weans, and the hormones stop overflowing my brain and sex becomes painful again which then makes my brain become less interested in sex. We have been dealing with this for 2 years, and at a point I was completely sex repulsed. Now I masturbate 1-2 times a month and we are intimate about 1-2 times a month.

A year ago we had a threesome with another guy. I felt back to my "normal" sexual self, and enjoyed the experience and ended up feeling more charged up at home with my boyfriend as well. My boyfriend felt hurt that I was feeling sexual attraction with someone else after not being able to for so long with him alone.

We decided not to continue seeing the guy as we had things to work through. We are now at the point where opening up has been discussed again, whether that means threesomes, or separate dates we are not sure. My boyfriend is understandably feeling lost as to what to do. I have started sex therapy but am only 2 sessions in.

Basically my question is, is opening a relationship a valid response to lowered libido in long term relationships? Besides this, our relationship is great and stable, but we both want to be back to being sexual beings. Also, I understand that my boyfriend feels hurt that I may be turned on with someone else and not regularly at home as easily, but am unsure how to ensure him that it is not his fault.

Let me know if I can answer anything! I have all the books we should be reading (Polysecure, The Ethical Slut, Come as you are) and don't want to jump into things. We just aren't sure if this is a good or ethical choice.

*I'd like to add, I've been interested in open relationships since I was in high school and am not only seeing this as a solution to a problem but also something that I believe I want to pursue either way.

TLDR: I (23F - bisexual) have a low libido with my boyfriend (24M - bisexual) and think opening a relationship may help spark interest again.