Its your body and you decide who touches you. But they get offended over such stupid stuff.
Still - its one of those 'no ground to give' issues ... you need to stand firm and have the argument on this. They need to respect you and your decisions.
Otherwise, what next? what is going to happen with the birth? with breastfeeding? with feeding, settling etc when the baby comes.
They need to appreciate that this is yours and your DP's baby .. that grandparents have a role, but its not as parents.