Good question! It might be due to translation or because the japanese seem to have a tendency of "keeping the gameplay and the lewds separate" In any case, i will agree that it feels like a hormonal teenager wrote most of the dialogue for games like the ones you mention because of a lack of respect towards story. And before anyone whines about "if you dont want porn in a porn game then go play a real game!" As op said, in moderation its good and can be quite sensual, but doing it like this is a mood breaker, all is good in the right amount.
Its a shame that dmm/nutaku opened up funds for the potential of western porn games and it feels like they had little to none to begin with. Might as well kept importing games from their site (AND LET SOMEONE ELSE TAKE CARE OF THEM *cough* goddamn sen pro).