This is probably one of those Head Smack (Doh!) moments, but ...
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Some adults are especially aroused by suckling at a lactating (milk producing) breast or just drinking mother's milk. New mothers lactate but others also lactate by natural or unnatural methods. "Wet nurses" used to be commonly used by aristocracy so the rich moms didn't have to do all the nursing on her own.  
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New moms can sometimes sustain milk production for long periods of time (years) by continued suckling and stimulation.  
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Personally, I prefer a nice, cold Yoohoo.