This game is heavily based on (infertile) insemination, might be a reason. The first chapter was literally about deflowering virgins, doesn't really sound like an opportunity for anal (virginity usually refers to the vagina and bleeding when taken, anal is "just" the first time). There are probably some opportunities for anal, especially with companions, but the game heavily focuses on the vagina. In my opinion it makes more sense, while some appreciate anal the more pleasurable place usually refers to the vagina, and the protagonist is all about giving pleasure.
Maybe I don't like it as much as other people do, similar in regards to foreplay, but I appreciate what the dev does. Often there is also the dilemma of "one person didn't get vaginal sex", "the other one didn't get anal", or other things, but here everyone is equal in that regard (I mean everyone gets vaginal sex), foreplay is just the exception.