When marriage between two guys happens, especially if there's enough age gap that could make either one or both of them insecure, you have Ohigara mo Yoku. A sweet, funny and same-sex "marriage" friendly book that softly fortells the story of how a childhood, painful, one-sided love turns into a fruitful marriage life later on.
Masao Sangatsu's art style gives it even more swooning touches, expressing the cuteness and sadness of her characters at the same time which is just beautiful.
One downfall for this title is the seme, his mood and decisions changes, his actions and rationale is almost baseless but despite all that, he still went into a, imo, very good conclusion about the whole thing with his partner.
If you're in for a comedy, supportive (straight friends), buffy and muscular men (Masao-sensei omg your art is awesome), light touches of drama and hurt, you gotta read Ohigara mo Yoku.