Dad or not, I've found it's all about your facial expression in determining whether the feeder finds someone watching to be creepy or not. That's why whenever I see a woman breastfeeding, I abruptly stop whatever I'm doing, open my eyes as wide as possible, stare directly, unblinking, and start slowly nodding encouragingly, so they know that I approve, while a big, toothy grin blossoms on my face to make it further known that I find the sight beautiful. Then I add a thumbs up at the end, once my grin is as wide as possible, just to make it all official.
Surprisingly I think it may actually accelerate the feeding process, because the mothers always stop breastfeeding soon after I finish non-verbally communicating that I find what they're doing to be beautiful, in a non-creepy way. Humans are amazing! 😍