unconventional relationships…?

I am a 32 year old woman who has just completed my Masters in Art and work in education for an art gallery whilst maintaining my own practice. My family live in another state but we still remain close. For the past 2 and a half years I have been involved with a married man. For the first year and a bit myself, and the couple were in a triangle relationship.
It was during this time that they became pregnant.
I was happy for them and we continued to see each other almost everyday. After the baby was born my relationship with the woman turned into friendship while my relationship with the man continued. Never were we seeing each other without his wife knowing and I had plenty of conversations in which we talked about this and she always reassured me that she was perfectly fine with it. She honestly still is to this day.
Now the man wants me to continue a relationship with him but she is pregnant again and I don’t think I am able to be so supportive for them when I am beginning to want a family of my own.
It must be pointed out that he is the most loving and generous and amazing man I have ever met. Past relationships have proved to be somewhat shallow compared to this one. And although it seems I am the ‘bit on the side’ I am the one who believes this even though he and she have never suggested or made me feel that. It is based on love. And they both are supportive of me, my art practice and my well being.
My problem is that I am in love with him but I am worried that this type of relationship is bound to fail. My mother had problems with it in the past and has come to realize (perhaps through her own failed 3 marriages) that love is love and no matter where it comes from. If you love that person then that’s the most important thing…yet she still subliminally suggests that if I were to have a baby on my own I can live with her and she will help me.
My friends and peers are now all getting married to their long term partners, having babies, buying house etc, and I am still living in a sardine tin of an apartment with no washing machine and none of the financial support couples benefit from.
Nor do I have ever the prospect of marrying him since he is already married.
Plus if I were to continue with him, would i live with them and be refereed to as the ‘aunty sarah to his children already?
I am worried I will not find someone and I am not even sure I would be able to share my space with someone since my need for having space is important thing for me. I sometimes like that he has his own home, but sometimes I wish he was living with me/ had someone to share with, cook for etc…
As this has been going on for a while now, my friends are no longer really good for advice since I am always in two minds, they too are in two minds; they say how good the relationship is and then they say how I deserve better or my own man/family.
I love him, but im not sure if my love is that strong to accept the on going complication of them having another child child and the unfulfilled desire for me to have a marriage or children (or me have children with him and then what kind of living arrangements would they be?).
I do care what others think but i’m mostly concerned with what makes me happy and I am confused as he makes me happy but the complicated situation does not.
Does anyone please have some advice or know of anyone who is or has been in this scenario? I would appreciate anything! Thanks xxx

