Since there are no answers here - I can’t do that (small boobs) but nursed all my kids and it’s not a sexual feeling. After the initial rush of nausea and terror I always got (and learned years later had a name and was not just insanity) it felt good in a nonsexual way, the stimulation just wrapped up in caring for the baby.

As part of sex, I’d say grabbing them myself feels good, is sexual stimulation but not the same as someone else doing it, not as good.