I'd like to adress a few points here:
1. Changing the NPC's sexual orientation is bad writing.
I thought about the movie "Kinsey" with Liam Neeson. There his character said that when he started to study the sexual behaviour of men and women he was sure of being a 0 or maybe 1 (0 standing for exclusively heterosexuall and 6 standing for exclusively homosexuall). Over the time it turned out to be a 3. That serves as an example that the sexual orientation or preference of a person can change. It's not unrealistic.
Creating a hole new romance plot for each NPC, "only" to create a believable evolution from heterosexuall NPC to bisexuall NPC will cost a lot of money, so that option would only stand with a romance DLC that needs to be bought or a new Add On like Dragon Age Awakening.
DLC: A party for the victorious, who defeated the Collectors. After the pressure of the suicide mission is gone people might realize that life is to short to be wasted and might see Shepard in another light. Remember only because Miranda admitted that she was in a relationship with Jacob doesn't mean she has never been with a woman. Only because she didn't say that she is bisexuall, doesn't mean that she isn't. I don't know about you, but I don't tell anyone of my sexual preference, the first time we meet, or even later. Why should I? A good reason would be that she didn't want to be distracted during the Collectors crisis. But now it's out of the way.
Add On: Even more possibilities. Shepard meets Kaidan and finally confesses his love. Kaidan asks for time to think about it. In the middle of the game Kaidan says that he thought about it and he doesn't as much as he originally thought. He is willing to give it a try. End of the game you get your romance and your love scene.
Just a few ideas of mine.
2. Create homosexuall characters for Mass Effect 3
I don't like the idea. If it would have been done in Mass Effect 2 and 2 NPCs were hetero, 2 were bi and 2 were homosexuall, we would have the dilemma of who would be the hetero and who would be the homosexuall NPC. Just imagine Miranda as the homosexuall NPC. How many people would cry out and demand a change? No, forget I asked. In order to satisfy as many as possible, every NPC should be made romanceable for either MShep or FemShep.
Still, some people feel uncomfortable if they realise that , Jacob for example, has the hots for you. Others might believe its to much of a character change and would damage the contuinity of the game. That's why I believe Shepard must allways be the one to start the romance. Never the NPC. Secondly the player must always know when he or she is entering the romance plot. Coloured text seems to be prefered solution.
3. I played as a FemShep and Tali was interested in a romantic way.
I agree. Not because of wishfull thinking, but because of what can be seen. khevan said that NPC's must be more defined, so the player cannot misunderstand. I agree with that. There is an indicator when a female NPC is interested in your character. It looks like a sexy pose. Sun Lian did it in Jade Empire. The secretary of Shiara did it. Kelly did it. Every time you see it you know. She wants you. Now play as a femshep, try to romance Tali (I know it won't work) and see what she does...
4. Is the sexy pose a solution?
For those who oppose the coloured text because just the mere sight of a possible gay content makes them uncomfortable, I had the idea of a "sexual preference check". During character creation were you decide for a male or female, soldier or biotic, you have to decide for your sexual preference too. In the game only the NPC's of your sexual preference would be able to have a romance with you. A few people sendt me a message. They thought it was degrading.
But I've got another idea recently. What if your Shepard will never be able to romance any NPCs, unless you strike the "sexy pose" the NPCs use frequently, during a conversation with your love interest. Let's say you'll have to press both the LT and RT on your pad. That way no one could say, I unlocked gay content by accident. That way your intentions would be clear and no one would think of it as degrading... I hope.