@SuperOniichan Its kinda complicated, his current appeareance is the culmination of all his ideals and fetishes from a woman, but he has still a strong attachment to his male character. .... You could say he sees his Dumbulf character as his masterpiece and ultimate goal as a man. He made his female character more like a joke and because bored but never intended to lock it in.
That is how I completely understand it because of what was said at the beginning of the story. His attachement towards his male character is mostly shown, when he sates for himself that he ddint gave up getting this body back. Even though he doesnt search for a way yet, because it is convenient for his mission. He is partly aware that she likes him and Marianne is not shown in any way to be aware of it.
Besides that the only possible reasons for him holding back is what others may think, but that is more a shoot into the dark.
Like I said the whole situation besides him holding back and confessing is more the typical "inexperience" (which is contrary how he acts to other situations where he doesnt act inexperienced/bothered at all) and his hope to get his male body back and be together with her in that form.
Therefore I personally think the author avoids the talking about it and going the "physical" part of this outside the characters. Which again is the point that is a bit out of character for them. Marianne can be explained because she accepts to be EVERYTHING he wants and therefore doesnt dare to demand more.
If I would try to describe it: They are like a married couple that lives together (and are aware of their feelings for each other), but the kissing and sexual things wont appear until they "confessed" towards each other. Which they avoid for various reasons. (Mostly he wants to be male and she accepts everything.)
PS wont deny that Marianne seems a bit "crazy" for me in that aspect.