Honki ni Natta Merryman (2010)

Blurb:
One evening while hanging out, Oku confesses to his friend and neighbor, Kanbayashi, that he is not attracted to women at all and thinks that he might be gay. One thing leads to another, and the two find themselves between the sheets sharing a night of passion. From that day on, though, the pair act as if nothing at all happened. Kan goes on to date a string of women while Oku begins seeing several different men. Three years on, things come to a head during Christmas break, with secrets laid bare between them. Where will things go from there? Two university students explore love, sexuality and friendship in this gentle coming of age tale.

Source: www.anisearch.com/manga/13997