This manga ebook was free this month on cmoa.jp! There's a minor age-gap (I think the girl is a young working woman in her mid to late 20's and the guy is a college student) and on top of that the age gap is in reverse of the usual pattern; younger male and older female. I enjoyed the sex scene in this manga because the guy actually got excited / flustered. Normally, manga sex shows the girl getting totally worked up and losing control, but the guy is totally cool and unaffected the whole time.
Download: Link Here!