You're not wrong to give up on breastfeeding.
YABU to give up because of your PIL's strange ways.
They are entitled to their opinion, they are entitled to voice, to are perfectly within your rights to ignore it completely.
I would have fun with it. Practice one liners that prey on their small minded views... I remember when I dressed my then toddler son in pink, and my extended family (not PIL's but their rellies) said I was "very modern" and "was I not concerned about his future"? I said "Oh yes, I am simply desperate to have at least one gay son, I mean, who else will go shopping with me?" (Obviously I don't give a shite either way, and do not stereotype people in my usual daily life) They soon shut up. They whisper when I see them now, but I only see them when someone dies so it's not too difficult to keep them confused, worried and afraid.
These are people that think that my one lesbian distant relative became 'that way' because she wore her big brother's jeans as a child. There is no reasoning with people like this, so just let it amuse you, when it ceases to amuse you, fuck them off... and if you can't do that, erupt to such a lioness extent that they zip it.