I'm sure that this has been said numerous time by other people, but my preference would be the ME3 approach with multiple straight, gay, and bisexual options for each gender. While it wasn't completely even, everyone got a choice and that's all I really am really asking for: a choice.
If the DA: I team went this route, they would probably be able to make it even for everyone. The main reason that it was so disproportionate in the ME series was because options carried over from prior games, which weren't equitable.
However, that being said, I'm perfectly fine with making them all bisexual from the start. It doesn't ruin immersion or cheapen the characters or strike me as too crazy of a prospect. If the option is all bisexual or no choice for some players (especially when others get a choice), then I prefer to go the all bisexual route.
"Not completely even" is an understatement. In ME3 a straight Femshep could easily be left with no options. I'm glad that there were options added in for non-straight Shepards, and glad straight male Shepard didn't lose out on much, but quite disappointed that the straight female Shepard got options taken away, particularly in the way that they did. But that's a discussion for elsewhere.
Again, I like the concept of six LIs of varying gender preferences. I just don't know if the execution would be done well. There's the budget issue rearing its ugly head again. I'd be concerned that even though there was relatively equal representation, you'd have uneven distribution of resources due to time/budget. So some might have more dialogue, and more polished dialogue, etc. Or they might all have lower quality. And yet, if it could be done equally, this way of handling things would certainly be ideal, as I said before. It seems like it would please the most people.
Again, in the absence of budget to make more than four characters love interests with proper writing and dialogue, either having them all be bisexual or player-oriented makes sense in the interests of fairness. But it's not ideal or even really a good solution, for reasons that others brought up that I, being a straight female, couldn't even have thought of. I guess it's true what they say about compromises: You just make everyone equally unhappy.