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It's entirely unrealistic to expect humans to coexist with a sentient sapient intelligent species of creatures and not have members of each having relationships with each other.
Factoring in the fact the more affectionate towards their trainer enables a pokémon to become stronger/more powerful in battle and the logical extension is a pokémon that sees its Trainer as its Mate battles to a level above this (humans seek any edge in competition, question is how far are you willing to go to get to the highest level).
Then there's the fact the devs themselves low-key confirm it continues to happen in their games, with a lot more little details surviving since TPC took over localisation as opposed to NoA's puritanical purging over anything remotely sexually suggetive in nature (they also added a longer skirt to the Beauty sprites from Gen II because "tHiNk Of ThE cHildReN111").
