Here's how I would do it in a series:
-Introduce one main/recurring character and do not
show his/her sexuality. Focus on establishing the character's personality and competence.
-In the second season, casually mention and show that the character is in a homosexual relationship with a minor character.
-Season 3 is the earliest point the marriage should happen and it should be a mundane affair.
Also, have gay couples in the backgrounds of scenes, like TOS did with people of color back in the day. Shoving the "accept gay/lesbian/whatever" message into people's face is more likely to backfire
than change people's preconceptions.