Going to jump straight to the questions as there is a lot going on. Is it possible they are quite content with you and not want males? Yes. How long would that realization take? Depends on how much self doubt the person has I suppose. there are certain things I have done in my life for years and years before realizing I really didn't care for it. You just kinda did it because it was there. So it depends on the person self awareness. Could there be more going on? Yeah there could, but you either have to trust them or tell them its going to stop and they need to stop being around an ex sexual partner. Or the other option is you can open the relationship and you can go explore your other desires just as they have.
Put simply they did something outside of the relationship and now you feel inadequate. That is what this boils down to. Thing about relationships is that they only work if there is trust. what this means is that you need to restore that trust. How you will do that is up to you, I cant say how you can go about that because I am not sure entirely which part was damaged. There are a good number of things going on, one being that they are 40 years old and unless they had a full hysterectomy, the baby timer is going to be ticking. it is not uncommon for Bi people (which they seem to be) to flip flop on their desires from time to time but they ALWAYS gravitate back to what they are with most often which would mean in this case you. There has to be more to your relationship than the sexual aspect and honestly that has to be enough in the end. If sex is such a large part of the relationship that it can cause such damage, there may be a much deeper issue than is in this listing. I am a MtF trans person and I have been in a relationship with one of my girlfriends for 7 years now (Ive been transitioning for almost 10). We have had sex probably a grand total of 5 times in our entire relationship and we are still together. Fact is my hormone meds make me not have a lot of drive and my dysphoria is so bad that I simply cant. She has a couple of approved ... playthings, with which she gets that element out of. that is how we work it. I trust her completely, I know her partners, I know what goes on, and they have guidelines to follow and they do. Her and I are based on a deeper connection, one that if she begins to have with the other person I already told her to go be with them. This makes the lines well defined and keeps them in check with what is going on. If your mate is going to hang around with this guy perhaps you need to find some guidelines to make it comfortable for you, need to open the relationship up so you can have side things too, or you may want to think about moving on to something that will be less damaging to you and more fulfilling. If you cant talk to your partner about it and make it right, that may be the best option. Sorry if this isn't any help, but there is a lot here that isn't seen and so I am trying to give you as much as I can. I wish you luck though.

Thank you so much for getting back to me. Firstly an open relationship definitely isn't an option for either of us. They have made it very clear that they want nor need anything else outside of what we have the problem is I have a hard time believing it. As for the baby timer thing they has a full hysterectomy 5 years ago and has had top surgery also. The only thing that hasn't been done is bottom surgery and we're both quite happy with it never being done. I also have 3 children from a long term previous relationship and all children still live at home so there wouldn't be the time or energy for anymore children although it's something we have discussed in the past . They have changed loads since we have met and I have to say that it's for the better. I think they were trying too hard to fit into a certain role of masculinity because of work etc they no longer work in that job and we've moved to a completely different area and they say that they have never felt more like themselves or more comfortable on their own skin than right now. They've reassured me that I've had more of them than anyone has ever had as they were always trying to fit into a role that wasn't them. I know they asked 3 previous long term partners to marry them but said that they were "doing what they thought men should do " . They have told me that they can't see themselves ever doing that again "as it's just not in them" marriage isn't important to me but I suppose it's just the principle of it that makes it sting a little. The fact that they are trans and have had such a hard time up until now makes me love them all the more not less but I suppose if it wasn't for them being trans then we might not be facing these issues. The damage that's been caused is me accusing instead of just asking for reassurance when I need it (which isn't that often anymore) They say they've given me every part of themselves parts that nobody else got to see and I don't doubt that and I'm grateful for it I think I'm just being too needy perhaps but it's taking the last of what we have

I don't think you are being to needy, I do think there is something that in you is wanting to think you aren't good enough for some reason. It could be the concept of the marriage thing, could be the other partners, it could be a whole lot of things, but for some reason you want to doubt what you have. Heck things could just be to good that your brain cant accept it. The reason the first matrix failed in the movie the matrix. Humans are feed on conflict, its what makes us grow and adapt. So if things are going wonderfully and there is nothing to battle we find something. I Do think there may be a bit of "this is man this is not" going on in their head and since they have been at it for so long that's a bit odd. I was married and am divorced and my current girlfriends know I will likely never marry again. Not because I don't love them, but because the divorce was horrible and not something I ever want to go through again. Even if I want to be with them forever, I won't risk it. Thing is that you are good enough for them. You just have to realize that in you. They are with you, which means that there is something in you that made them want to be with you. In the end you will have to be able to learn to trust them as it will be the basis in the end. I think both of you should go see a therapist, if not together then independently for sure with some sessions together. You both could use someone to talk to from a few different angles who could navigate the complicated areas like the trans thing and all. May help with the insecurity, the "what is masculine", the not feeling themselves or feeling like themselves, so on, there could also be some fear of falling into them. losing who you are and becoming part of them. There are just so many possibilities, but something is missing or being held back. Which side is doing it, what that would be, I have no idea and that is why I suggest the therapist. they can usually find it. Trust is scary, but if you can find it, I think you two will be fine.