I think he is wise to ask you to consider the relationship you have together, and put this first. Neither of you are spring chicken. He is 46 and have older teenagers. He has been able to be there for them, and it is a massive difference between becoming a mum/dad at 30, compared to at 43 and 46. Having a newborn is exhausting, and it does change your relationship.
How keen on children are you really, if you have lived to the grand age of 43 without wanting kids?
Also, he may want to protect you from more heartbreaks, another miscarriage, or also the statistics of older mums and problem pregnancies or risk of children having special needs. Or he may simply not want to start again with sleepless nights, nappies, feeding, crying, and the whole cycle of supporting children through school etc.
I am 42, my kids are 11 and 8. I sometimes feel I am too old to deal with the issues they bring.
Think about it, you will be what, 52 or 55, dealing with preteens, and close to 60 with a stroppy teenager. At 70 your child will perhaps leave secondary education and embark on Uni life. At 75 they graduate, at 80 you become a grandma.... How much support will you be able to offer your child through it their life? Will they have a sibling? Or will they be alone in dealing with two elderly parents in their late teens or early twenties?
Why not embrace the love and the great relationship you have?
What is the alternative? Going it alone, get pregnant with a virtual stranger, have a one night stand, and in that way ensure that your child grows up with the support of ONE parent only, and should the burden of an elderly parent totally on its own.
I am speaking from the perspective of having an 86 year old dad who had a stroke 11 years ago and who has spent the time since then paralyzed and in a wheelchair. And a mum who developed Alzheimers at 70 and who is now 78 and has been in a care home the last 3 years. My perspective is very coloured by this. I have had a true nightmare sandwiched between elderly and care needing parents, and the needs of young children. I have had a very helpful and loving husband in this, and also a sister to help me shoulder the burden and the responsibility. My mum was 30 when she had me. You will be nearly 15 years her senior when you have your first baby. What guarantees do you have that you will stay fit and healthy?
Is it fair on the child?
None of us have guarantees, but why take the risk?