Unfort this is one of the reasons relationships break down during wedding preparations. Guests think they would love to go and its Ok to challenge the hosts on how they want their wedding so they themselves guest can enjoy it more.
If you are invited to their wedding - they know you have a baby. If they know you well enough, they know you are breast feeding and they still have not offered for your baby to come with you.
Inviting babies is not usually about seating plans although it can be. It's often about them crying for one. Parents think, we'll I'll take them out if they cry...great but they have already been crying in the venue in the first place before you took them out. And some parents might whisk a child away at the first sniffle, others will take a while as they are having so much fun and hope they stop.
And then there is the baby that starts to cry during the vows - you can't take that back and it would appear on the video so your only option would be to avoid this scenario by taking the baby out of the room at this bit just incase...and then you missed the bit of the wedding the couple want you there for in the first place.
The next issue is if they say yes to you it might upset their other friends. Friends whose breast fed baby does take a bottle so they have arranged childcare. Friends who can't breast feed so they baby always had a bottle and arranged childcare.#
Then there is also the issue of drunk people around a baby. Are you going to be holding this baby the whole time during the reception? Leaving them in a pram and never leave their side to make sure some happy chappy doesn't spill their drink on them or try and hold them? Everyone loves a baby and its going to be a distraction for people who are going to be trying to interact with them.
And then there is a the venue - maybe they don't allow children in an area with alcohol.
You have written this post from how this wedding invite affects you - you don't want to stop babies routine etc etc. You want to enjoy the wedding etc etc. But its not your wedding - its their wedding. Don't put the couple on the spot by asking them this - yes they can say no but it will be awkward for them. They might resent you - your partner might resent them etc if they still say no babies and it means your partner has to go alone.
Really I think you only have one option - decline and explain its because you are breast feeding. If this is something they have not thought about and really want you there....they will say to you - please don't let that be the reason you have declined you are welcome to bring the baby.
And if this does happen, ask them to ask the venue if there is a room you can breast feed in.