I mean, it's actually pretty easy to explain why some girls never had any real experience with a man. Let me make a short list:
Lucy: Was bullied by every human being she encountered that is not a household member or friends with Sandra
Alice: Was always seen as "too small" to be considered a real girlfriend, also never really had the chance to enjoy her time because of the orphanage
Sandra: While guys hit on her she never had any interest in men, mainly because of the things that happened to Becca and that men usually only wanted one thing with her
Madeleine(Maddie): Never really cared about other dudes because she was more focused on her body and showing that even a small girl can outshine men in sports. Her sexual feelings started when MC showed her his pp.
Yvonne: Was always in Sandy's shadow, sometimes seen as the lesbian girl that isn't a lesbian. Didn't care about guys anyway because Sandy was more important ( in a non sexual way and to enjoy her voyerism later on )
Arianna: Had boyfriends, all of them turned out to be gay though, which is very unlucky, but it can happen. The most intimate contact she had was with Yvonne
.
Sun: Too shy, especially because of Soo-Jin acting like she acts, MC helped her out of her shell and she fell in love with him.
Soo: She had boyfriends but only played with them, saw how happy Sun is with MC and started to get jealous and feel the need for more than just playing with boys ( and not giving them anything )
Tanja: She is kinda like Soo, only in a meaner way. Never bothered to care for guys because she was too much of a bitch to find real love.
Melissa: See Lucy. She is also a victim of getting bullied and having the scar in her face didn't help as well.
Now, let's talk about the other women that are much older and should be more experienced. I'm leaving Becca, Mary and Elaine out here since their story is already explained enough.
Fiona: She always had to prove herself against men, everyone told her women can't achieve the things she achieves. Tried to make a boy toy out of MC, found out he's not that easy to manipulate and opened up to him. Now they are friends with benefits with the occasional doubt in "I don't want a romantic relationship with you." She enjoyed the relationship to a woman more ( Mary ) but is now seeing that at least MC is not a pig.
Dianna: Her husband never really appreciated her for what she is. While they tried to have some intimate contact it never happened because of how uneasy she felt around him, never getting the feeling that she is a real woman.
That leaves a few women who are not yet in the harem to be determined. I can already spoil that Kiara is a bit too much like Lucy. Not that she got bullied, it's quite the opposite, but since she is so smart it's not easy for her to start a relationship. Also, being a doctor is her true love. Maybe MC can change that.
Anita is on a whole different level and I'm not sure where I'm going with her in terms of romances. That's something for the future. Same goes for Sun's mother.