OP the word you were looking for was trawling, not trolling.
I do understand where you're coming from as it can sometimes be odd that someone has taken time to search out a post about parenting issues when they aren't parents now or yet. BUT, I have just one child. Does that mean I can't have something to say on the subject of siblings? Because I have two of those and many,many stories of issues around siblings and what my parents found useful in dealing with those!
Or can't I comment on teenagers because my DS is only seven? Because I have neices and nephews ranjng from fourteen to twenty eight so have seen my sisters and SILs deal with teenager years (and been at the 'grunt' end of a conversation with an uncommunicative teen neice or nephew many many times!). Perhaps my family's or my own way of dealing with those things may be the one post that actually helps someone.
I haven't read the particular breastfeeding post you mentioned where a single man joined the conversation so I can't comment specifically but he may have a friend or sister who has had problems or who has given him a valid opinion though their thoughts on the subject.
You're right, the site is called Mumsnet. But you, yourself, use the word parents...Maybe Mumsnet should be renamed Mums, Dads, Aunties, Uncles, Grannies, Grandads etc Net. I would say 99% of the people in the world are related to a child in some way and, for that reason, their views are valid in my opinion. Just because they aren't a parent doesn't make their view worthless or sinister. I couldn't BF my son, but I still have an opinion on the right of a woman to BF when and where she wants to because I can out myself in her shoes and I have friends who have BF. I feel my opinion on the subject is just as worthwhile and important as a mum who is or has BF.
As for your point about not going into a singles site to offer an opinion even though you have, obviously, been single in the past...I would, if I stumbled on the site and someone was going through a problem I had encountered in my single years. Why not? We had our son through IVF. I used to regularly revisit the site that supported me when we were going through what can be a rollercoaster ride of emotions and would regularly post to offer help, support and encouragement. I don't see the problem to be honest.