Too short to truly give good feedback.
The concept is okay, everything else could be improved by quite a lot.
The writing is kind of obvious that it could be better, the introduction is too straightforward, with NTR and corruption I don't recommend a visual novel style but rather a free mode and the mind control feels a bit too strong.
Ah well will do a tiny favor and correct a bit the the starting lines:
-You are Billy Connor, a normal guy who lives in this bible-thumper town called Chastity Spring
-And this innocent, plain but beautiful girl is your girlfriend Jenna
-You have been in a relationship with her for years and you both feel ready to take the next step, which is marriage.
-If only marriage is that easy... there are a lot of hurdles to pass, which is why you both are feeling insecure at the moment.
-To solve that situation you decided to get a marriage counselor and here is where the story begins
-You are in the office of Dr. Lili Thesire, a counselor that specializes in marriage and family consultation, she has been recommended by your stepmother.
-I have been waiting for you Mr. Connor (finally a perfect line in the game!)
-I'm Dr. Lili Thesire, I'm very glad to make your acquaintance (don't truly get why you want to add the girlfriend part when she isn't even present).
-She wears such a revealing outfit... I wonder what was Jennifer's reaction when she met her yesterday...
-I have heard already what your mother and girlfriend wanted to say and now it's your turn (it feels much more natural for me this way)
-You tell her your worries.
-I know it already!
-Both of you are so naive, inexperienced and boring.
-Do you like what you see Mr. Connor?
-Which boobs are better, mine or your girlfriend?
-W-What are you doing Mrs. Thesire!?
-Don't you want to touch them Mr. Connor? to feel how soft they are?
-Please stop it Mrs. Thesire!
-You are no fun, in fact this town is as fun as a rock, but well... good thing that I'm here to solve it.
-Solve it? What do you mean?
-Worry not Mr. Connor, the party is about to start and you are already in.
-No way! I'm out of this crazy place!
-Hahaha, did I say that you had an option? I fear that you don't young man, by the way, don't you find my boobs very... enticing?
-W-What's happening?
-Shhh, just focus on them.
-Good night boring Mr. Connor
-Ah!, sorry Mrs. Thesire, I feel like I did daze off for a moment, I'm sorry.
-Worry not Mr. Connor, it has been quite an intensive time and I understand, because of that I will ask the question again, where are you and your girlfriend going to live from now on?
-We will live in my stepmother's house.
-Why is that?
-My stepmother can teach my girlfriend how to be a good woman and even more I can be of help in return by helping with the chores.
-That's good, I have had a pleasant time talking with you Mr. Connor but I think that our time is over, will see you again in the next visit.
-Two days later you move to your stepmother's house with your girlfriend.
Here is where I end the correction and a few text changes so that the flow/wording feels more natural or to give more personality to Thesire.
With how the story is flowing after that scene I feel that it's a mistake. I would guess that in the first session the influence of Mrs. Thesire should be that the MC would feel that his girlfriend needs more confidence, she needs clothes that aren't as prude as what she is wearing. Basically I feel that the MC at first should be used to support the "corruption of his girlfriend". The shower peek is pure fanservice and that the MC is seeing his sis... I mean stepsister as a girl directly feels too fast, rather the MC should think about how his girlfriend should learn a bit from his stepsister.
Basically gradual corruption against fast corruption. In fact about the training of your girlfriend the MC shouldn't have an option at first, the natural thing would be to follow Dr. Lili idea because she is a professional and you had a good feeling towards her based on your first visit.