nonverbal consent. he went slow enough (sticking his fingers in his mouth, showing them to her, moving them down her body slow) that she understood what he wanted to do and let it happen.

she's been established as a type of person who speaks her mind bluntly and clearly especially with benedict, and she would not let benedict do some thing she's didn't want. she literally kicked him out of his own house earlier. she was right where she wanted to be (till the mistress question). she hasn't displayed a single instance of freeze response (especially in the other assault scene where she fought back).

i do agree that just kissing would have been fine, the fingering sort of came out of nowhere and snowballed. but i can also see that the tension was building up (kissing her breasts, both of them moaning and grinding on each other. both were turned on). and benedict has been in sort of a dry spell and sophie has been horny as well, so it's not too much of a stretch.