Other author going to interrupt here so don't mind me but I wanted to give my opinion about Snape being a bottom and a "bottom" in general.
1) Snape's life is a tight rope act. He always has to be on be in control know what is going on sort of thing. When with a partner and in a relationship Snape is allowed to not be this 100% control guy. His love and sex with Harry allows him to give up to be allowed to be pleasure by someone else. Not FUCKED! but pleasured and touched and made to feel things he can't feel every day when he has to be on.
2) Snape gets fucked in canon so why not translate that to the bedroom.
3) Harry is not this submissive little boy. He does get played to a point and pointed in directions but not always. He certainly pushes back against Snape of all people"No need to call me Professor, sir" or whatever the line is. Harry yes is manipulated into certain positions in the story but he chooses others for himself. From the first book choosing to tell off Malfoy to the last when he realizes DD keeps secrets he doesn't. He has that natural quality that is found in a leader.
4) There is power in being a "bottom". Allowing Harry to enter him to gain pleasure from his body. Snape can still be in control and in charge. Faster, slower, harder, there, yes, no, take me. Bottom isn't always lower on the hierarchy scale, it is possible to be equal or above.