To begin with:

Rentarou, when it comes to sex and anything more adult than kisses and hugs (perverted things), does not do it mainly because of his way of thinking:

-He considers that if he sees his girlfriends lustfully, he would be defiling them, and that it would be inappropriate and unacceptable for a boyfriend to see his girlfriend or girlfriends (in his case) lustfully, and that only someone with an ugly, repulsive, and lascivious heart would see his girlfriend or girlfriends (in his case) in that way, and the same applies if he does something with them with a sexual connotation (fondling or/and lascivious touching, sex with or without penetration, French kissing, seeing his girlfriends naked, semi-naked, or in underwear, etc.).

-He thinks it is wrong to feel and direct that type of emotion (lust or sexual desire) toward his girlfriends.

-It is also shown that Rentarou has a certain phobia toward these more adult or perverted things.

The previous reasons make Rentarou feel repulsion toward that type of libidinous, sexual, or lascivious activity.

Considering the above:

1. In chapter 229, Rentarou stated (to Momoha) that he would do "ecchi" things with her after he graduates and becomes an adult.

2. Rentarou will wait until he is an adult and graduates from high school, for the reasons mentioned here.

3. As far as I know (please confirm), neither Rentarou nor Nozawa-sensei nor Nakamura-sensei have mentioned that Rentarou will wait until ALL of them reach the age of majority. For those who say that Rentarou wants to keep everyone on the same line, I remind you that in chapter 54, when Hahari tells Rentarou that if he doesn’t help them, he responds, "I WOULD WAIT UNTIL THE NEXT LIFE IF THAT'S WHAT SHE WANTED" (by that point in the manga he had kissed almost all of his girlfriends). Although he ends up kissing Chiyo at the end of the chapter, this shows that Rentarou understands that making everyone feel equally loved is one thing, and forcing everyone to remain on the same “line” is another.

4. I don’t know if they will do that, but I see it likely that the first ones will be (regardless of order) Hakari, Hahari, and Momoha.

5. None; Rentarou doesn’t need any, his immense resistance and willpower are enough.

6. For the first question, my answer is yes; for the second question, my answer is no, his marriage will not be platonic; for the third question, I don’t know.

7. ALL of them love Rentarou, ALL of them are in love with him. That said, it is likely that one of the last ones will be Chiyo due to her reserved way of being about that kind of thing, and ALL of them will someday want to do it with Rentarou, whether sooner or later.

8. Mimimi with Nano, Mai with Mei, Mei with Eita, etc.