I feel compelled to explain the importance of the fact that this character in particular is engaging in anal sex for those not familiar with Kantai Collection and the character of Houshou.
As you may or may not know, Kancolle is about anthropomorphic WWII warships that appear as cute girls. The original light aircraft carrier Houshou was commissioned in 1922 as Japan's very first aircraft carrier, and had the distinction of being the first aircraft carrier in the world that was built as an aircraft carrier (all other carriers before her were conversions of ships originally designed as some other type of ship). She spent the majority of WWII proper in Japanese home waters serving as a training ship for pilots training to operate on carriers. These historic aspects are manifested in Houshou's character by her being very motherly and domestic. One of her secretary lines in the game is literally the stereotypical "what would you like to have first?" type of Japanese wife lines. She is a very reserved and quiet character that does not at all seem the type to be anally receptive, so to speak. Hopefully you can realize the importance of the fact that this doujin is about Houshou having anal, and why it makes it roughly a thousand times better than it already is.
tl;dr : demure literal waifu character responding to the prospect of anal sex not with disgust but curiosity is hot as fuck