This is the third one I read aboutt a Titan doing that but in the other two it was Titan Eren molesting and the captain was his uke. I assumed this was one of those. I prefer those two although I like one better then the other but can't remember which one I liked best.
I once read Eren X Titan doujinshi and I figure their sex would work something like that http://www.mangago.me/read-manga/shingeki_no_kyojin_dj_what_reflects_in_those_eyes/mh/c000/14/