Taken at Both Ends - Deeper Space, #3

When Ash asks his commander to invite a stranger for a threesome, there's no way he can anticipate the big, pushy mountain of perfection Finn finds. P'ri is massive compared to the small human, and when it comes to sex, it turns out he is proportionately endowed.[This book depicts rough sex, unrealistic physical situations, an inhuman partner, and an officer fraternizing with a subordinate.]