I wonder if it's just that you two aren't really ready yet for other people to be so heavily involved in your relationship?
My SO and I for various reasons relating to lack of practise, past relationships etc etc have some work to do on communication. Work that is taking ages - it's hard to do and we're both busy! Until we are able to communicate openly with each other, I wouldn't be happy about either of us developing new, intimate relationships.
I'm not talking just about sexual relationships - any that would take up lots of time, effort and large involve emotional adjustments. I wouldn't have a baby with him just now. I also wouldn't get a puppy or a new dog.
It just seems like doing so would make an already difficult job harder?