If it makes sense and adds to the game, do it. I'll give some examples of how not to do it:
1) If it doesn't make sense for the character. For an example, Ashley in ME1 was very classic Christian, and it's safe to assume that 200 years from now there will still be a large number of Christians who believe homosexuality to be wrong. Ashley gives this impression throughout the game, and only flirts with BroShep, never FemShep. Suddenly making her bi for no reason in ME3 would arguably betray her character.
2) If there is no difference between the sexes. I mean, I get it, people are people and genitals don't matter when it comes to love, yada yada. But no one can honestly tell me that, both physically and vocally, a bi person never treats women differently than men, and vice versa. If we have the option of romancing the same character with either a male or a female PC, then I expect differences (if only subtle) between those encounters. I also expect that LI to give signs that he/she is bi regardless of us romancing him/her, so that it doesn't come off as a straight guy going gay for just my PC. Once again, it feels as though it's betraying the character.
3) If there is no social implication. Look at it this way: In DAII, Hawke can be a lesbian, right? So, her entire family dies off minus one or possibly two people. Wouldn't there be some sort of consequence in the eyes of Kirkwall for the last of the Champion's line to not be able to have a blood heir? Who would inherit the estate? Wouldn't there be people fighting for Hawke's blessings in the very likely chance she dies in battle? Why is everyone indifferent to the fact that their Champion chose to knock boots with a Dalish elf living in the alienage with all the social prowess of a plank of wood? If Hawke is so famous, why does NOBODY have an opinion on her love life?
DAII's romances built individual character interaction, but that was about it. There was so much more that could have been done with eah and every one, and much like the rest of the game, it just fell short. LI's, to me, are a huge waste of potential. Remember the camp in DA:O, when Morrigan and Wynn had very vibrant opinions on your relationship with Leliana? I cracked up when Morrigan bluntly told me (with disgust) that I "wouldn't stop smiling", and I felt an emotional obligation to stand up for Leliana when Wynn questioned whether it was wise to be with her. That never really happened in DAII, outside of some isabela banter that never extended beyond sex jokes.
Anyway, point is, the "bisexual" part of the romance shouldn't be the hot topic. The implementation of the entire LI, regardless of orientation, should. If Anders is only straight but that straight relationship can be more fleshed out, then I'll take it. If Anders is now bi but each side of the romance gets its due, then I'll take that. Compromising one or the other just to please a minor amount of heavily invested fans with a half-hearted copy/paste isn't going to make anybody happier.