We're not a step or blended family but this question is a really good one so I want to stick my oar in, pls disregard if I'm posting out of turn .
I wouldn't be comfortable with that at all (even being asked it would make me so unhappy). We were ttc for 8 years, for someone else to carry DHs child it would just break me (plus I'm sure if other kids are involved and there are regular visits, I'd have to be a step parent to a child my husband conceived while married to me, it's just too hurtful).
On the other hand I can see why someone would want the same father for all their children, but I think you have to let go when your ex has moved onto a new family (asking a single ex for this isn't that unreasonable).
I also second pps concerns over not paying child support, if anything it muddies the waters (and makes it a paid donation which I think isn't too legal in Australia).
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