This is just a short theory that I came up with ever since I first saw Momonosuke all the way back on Dressrosa, especially the way he was acting was very suspicious. Maybe it’s just me but no kid of that age would be attracted to a grown women body.
Also, looks like he is actually enjoying it. There are many scenes where we see him doing this with both Nami and Robin. I always believed deep down in my heart that this little runt is actually a full-grown perverted man but just turned into a kid somehow when he encountered Kaido in Wano Kuni. The reason why I really started to believe him being a grown man was when he said that he remembers meeting Gol D. Roger when he was a kid. And I believe he was actually telling the truth.
Read the dialog above, Momonosuke specifically says that he too has made the acquaintance of Roger, he just doesn’t remember him since he was very young. I believe that he was at least around his late 20’s (28 to be exact, the power that turned him into a child is probably 20 years reduction) when he encountered Kaido. You guys might ask, but we clearly see Momonosuke as a kid when he remembers meeting with Kaido in his small flashback. And you guys are right he was young, just look at his expression on these panels. He clearly looks like he was frustrated being so weak and helpless to do anything. Just look!
That scene probably happened right after he was turned into a child and he was completely helpless to stop Kaido from killing his parents. Also, I wanted to point out the relationship he shares with Kin’emon. Kin’emon is probably his friend/guardian before he was turned into a kid. They probably used to pick into women bathhouse together also the reason why Momonosuke keep making Kin’emon jealous by grabbing both Nami’s and Robin’s boobs.
I know this theory is a bit out of nowhere and we don’t really have much evidence to support this theory, but, I just wanted to share this with you guys.
Hope you all enjoyed this short yet entertaining theory of mine.
*Theory by iamkushank