That happens in the game.
To get the best outcome from Sof you need to turn down her advances and they have a heart to heart where he tells her she doesn't need to offer herself to him.
Eventually they get together under proper terms.
Anybody that has taken her up on the blowjob offer has just confirmed to her that's all men want.
It should be noted that is the recommended choice in the Harem route, which happens soon after Sofi arrives at your house and offers something in return for giving her a place to stay.
However, the Walkthrough recommends taking the "Play along" choice in The Forbidden One route (i.e., Ada is your primary interest.)
Out of curiousity, I tried both choices for Sofi in The Forbidden One route and while neither choice seems to affect that Ada-primary path, Sofi's revelation and gratitude is lessened (i.e., her discussion scene on their "harem" arrangements is cut short), she gives you that obligatory blowjob in Ada's room and doesn't visit you in the hospital late at night. So, "Play along" with Sofi (instead of initially turning down her advance) is apparently a way to distance you from Sofi for an even clearer path to Ada, I suppose.
