The situation you encountered is the bed you made to lie in yourself. Seems you pushed too much on the Lust route! In my main playthrough Clara (not Amy)
This game has a clever design! If you play a total horndog and take choices into that direction, odds are Clara (and Amy and Betsy too) will become loose sluts over time. If you like/love Clara, Amy or Betsy, you can get loyal girlfriends, even slutty but loyal ones, if you choose wisely on the "Lust" and "relationship" choices! For instance, in my main game, Clara currently is around 5-6 Lust points, is quite the kisser and lets me touch her and stopps advances of others on her own!
That is the result of choosing a mixture of Lust and relationship choices only(!). I tested with different choices and this has impact! In another playthrough, Clara likes me lots, but is obviously on the way to a loose slut, since I took "Lust" choices mostly. Conversly, you can get a extremly loyal, but Nun-like Clara, if you go overboard with "relationship" choices.
To get an open, later slutty in bed, but loyal to you Clara, you have to work on your relationship. Show that you like her, don´t do too much with other girls (blowjob at the party is okish, but do not try to land with Bean when that possibility opens up for example) and take many, but not too many(!) Lust choices to open her up. As said, my main game Clara opens up nicely for me, but blocks the advances of others on her own.