If the marriage system in any way relies on user input and isn't pre-scripted then there shouldn't be any issue in giving same-sex options. Not every game has to be like Bioware's more heavy-handed "everyone's bisexual" approach. I mean, it's not a lot to ask really.
Then again I've never really shown much interest in the Fire Emblem series, so this is another reason not to.
Then again I've never really shown much interest in the Fire Emblem series, so this is another reason not to.