Sounds like you are using Sexlab Aroused Creatures. Creatures take time to react and register your character as being in the vicinity, especially depending on the diistance that you have set in the options for checking if the creature is a valid actor and for the distance for the creature being able to sex your character. Arousal is also integreated into that plugin so you might need to set arousal for both the creature and yourself to initiate sex to a really low value or zero. If you want to submit to hostile creatures, I suggest downloading either Sexlab Submit or Sexlab Defeat. They both work with creatures to an extent. They can be a bit buggy when surrendering/submitting but I believe that issue lies within script limitations in the game engine.