I could only guess at the developers true motivations for doing the things that they do, and while those would be based on things that we have seen them do and things that they have said, they would still be guesses and not facts.
There are a number of issues the developers have to over come:
1. Time compression:
Generally romance between two people with little or no (recent?) knowledge about each other takes time to progress from first meeting, through getting to know and feel safe with each other, to having some light forms of physical/sexual intimacy, to finally having sexual intercourse for the first time. Remember the current elapsed time in the game is only 12 days of intractable time between the father and daughter, and the developers have had to fit weeks if not months of romance into that short time.
2. Innocence and the lost of it, both mentally and physically:
For many females both of these things are a big thing, especially the second one. Many seem to find the idea that someone in their late teens can still be innocent (especially mentally) unrealistic in these modern times of smart-phones / social media / on-line porn, but maybe those people are only projecting their own experiences / biases onto the D character. I know a number of females (and males for that matter) that use feature phones and that don't buy into social media, I have no idea about the porn part as that is generally not a topic that comes up socially.
3. Players and Patrons
Make a game that people looking for (semi) interactive porn will be interesting in playing, and a project that people will want to donate their money to so that the developers can spend time making said game.
4. Characters that fit the core premise of a father dating their daughter:
A. Father:
Based on what little we have been told about the time between his break up with the mother and the reasons for the breakup we can infer than he has had little or no sexual relations in that time, and that he found the mother physically sexually appealing. We could wonder why he isn't hitting on the receptionist who flirts with him but maybe he isn't ready to move on yet, or thinks she's just being friendly, or that he's worried about have a (sexual / romantic) relationship with a co-worker, etc... but we don't know for sure because we haven't been told.
We can also infer that he loves his daughter but has an outdated mental image (both physical and behavioural) of her, which is confronted when he meets her again and discovers she has grow up (and outwards) to physically resemble the mother. The subconscious mind is a cruel thing and can react to visual signals we wish it didn't, it also doesn't help that the player gets to chose to to direct the father towards being sexually interested in the daughter which is hardly surprising in game with this core premise.
B. Daughter:
For the daughter to lose her innocence (mentally and physically) the writers need to come up with a reason for her to still have it, and those reasons need to hold some water. They chose to use:-
i. A manipulative mother who is keeping the daughter like she is so that she is easier to use as a token to further her new partners business, which in turn furthers her own life style.
ii. A religious school / upbringing to reduce the chances of exposure to sexual experiences/knowledge, not to be confused with Reproduction Studies that may be taught in such schools.
iii. A slutty best girlfriend who's sexual actions and advancements are used to make the daughter weary of doing such things herself, because the mother (a person the daughter currently trusts) keeps telling the daughter the girlfriend's actions are wrong.
For the daughter to consider the father (who she's been told cheated on her mother) suitable as a potentially date-able person, she first has to come to terms with dumping her existing boyfriend and going against her mother's wishes, she has learn to trust the father then come to the decision that he may be worthy, and then there's the whole incest topic to over come for both of them let alone the subject of introducing her to the world of sexual interactions.
We like to try to associate words like believable and realistic with the story-lines of porn related interactive fiction/games but because they generally requiring sexual interactions within a very short time frame then by their very nature they can't really be that believable or realistic in a general sense can they?